iMac doesn't go past loading screen

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

    Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals.
3. Resetting your Mac's PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences.
6. Start the computer in Safe Mode. Test in safe mode to see if the problem persists, then restart normally. Also see Use safe mode to isolate issues with your Mac - Apple Support.
7. Use Apple Hardware Test to see if there is any hardware malfunction.
8. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list.  Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
9. Repair permissions on the Home folder: Resolve issues caused by changing the permissions of items in your home folder.
10. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
11. Download and install the OS X El Capitan 10.11.6 Combo Update or 10.12.6 Combo Update or Download macOS High Sierra 10.13.6 Combo Update or Get MacOS Mojave now from the Mac App Store as needed.
12. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
13. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

     1. Select Disk Utility from the Utility Menu and click on the Continue button.

     2. When Disk Utility loads select the drive (out-dented entry) from the Device list.

     3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.

     4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)

     5. Click on the Apply button, then wait for the Done button to activate and click on it.

     6. Quit Disk Utility and return to the Utility Menu.

     7. Select Reinstall OS X and click on the Continue button.

Try running First Aid from Disk Utility while in Recovery Mode. Make sure the "View" is set to "Show All Devices". It would explain why you couldn't select anything but APFS when erasing it. Your symptoms sound like a drive going bad.

There's potentially a hardware problem lurking here, though whether it's with the storage or something else?

I'll assume you've gone through Kappy's list and the other suggestions here, and have not had success.

In addition to those previous suggestions, remove all external devices except for the keyboard and mouse, if you're using those. All disks. All hubs. All USB cables. Everything, save keyboard and mouse if present. Thunderbolt, FireWire, or whatever other I/O ports might be available here, depending on exactly which iMac model. See if that works better.

If that fails to work better, also then try configuring one external wall-powered hard disk or SSD device—wall-powered or powered-hub if it's a hard disk, and not a bus-powered hard disk—and install macOS on that external device either from Recovery or from a locally-built bootable USB installer—and then see if that external storage boots and runs any better.

Bus-powered hard disk drives can draw a lot of power particularly at spin-up, and can tip a marginal or underpowered USB configuration over into a fault, or into deep flaky. There can be power problems in even fairly small USB configurations too, as well as macOS problems caused by flaky USB devices, and by bad USB cables or bad hubs.

If the internal storage is found to be the likely problem based on the above testing, then plan to replace it with either another Fusion configuration or (preferably) with an equivalent or larger SSD device.

The Fusion drive is suspect, though whether that might be the drive or some other part of the I/O hardware?

The bootable USB flash drive with an installer is a basic test of whether the system will boot and run on any device; from a USB device, here. It'll let you clobber the internal storage in various different ways and to then run some boot tests, if you're inclined. And you won't have to keep downloading macOS via Internet Recovery when testing; not once that USB bootable installer has been built. This is a "will it boot and run?" test.

Using the bootable drive to then create and install a separate and external macOS environment on an external storage device will test more of the system, though not all of it. This external storage will also provide you with a path to get this system working again at least temporarily, pending further work on the Fusion storage or with whatever is wrong here. Or a decision to replace the iMac, if it's old enough and if it looks like the hardware has failed. This is a "get something working" test, and with the option to get your files restored, pending repair or replacement.

Three or five terabyte external USB hard disk storage devices are comparatively inexpensive, and still generally useful. And the external storage can be repurposed later as a Time Machine storage device or most any other usage, once you're back on a working configuration.

If the Fusion storage appears faulty during testing and you do decide to repair the iMac and replace the internal storage, I'd go SSD here and not Fusion, nor a hard disk drive.

An Apple Store will usually run diagnostics for free, if you're inclined to see what the folks at Apple think about your options here, and an estimate for a repair—should this problem be caused by bad hardware.

Continued from above:

14. If none of the above helps then see How to Downgrade macOS High Sierra and macOS Reversion- 

      How to Downgrade from High Sierra.

15. If you get here without success then it's time to make an appointment at the Apple Genius Bar for 

      service. If you need to find an Apple Store - Find a Store - Apple.

You should be able to click on the top level of the drive which is named "Fusion Drive" and click "Erase.

It might be easier if you boot into Internet Recovery using "Command + Option + R" which will install Mojave from the internet.

The previously-mentioned bootable USB installer can be created on an ~8 GB USB flash drive using another Mac, transferred to this Mac, and booted.

Removing and replacing the internal Fusion storage does involve disassembly of this iMac, which you can either choose to learn about and perform yourself, or a local Mac repair shop can assist, or maybe the local Apple store might be willing to perform.

If you don't have an external USB storage device available, it won't be feasible to operate from external storage. That can be a stop-gap pending internal repair, and the external storage can then be repurposed for Time Machine or other uses once (if) the internal storage is replaced.

This assumes that the trigger here is the internal storage, which is far from certain.

Performance degradation and weird behaviors can be caused by flaky hard disks, and a Fusion configuration is some macOS software merging a dinky SSD and a big hard disk into a single device, with some fancier file-shuffling support added.

Do you have a backup of your data? I would try reformatting the drive selecting the top level "Fusion Drive".

Goedemiddag,

I think your drive is dying.

Which Imac do you have (Apple menu->About this Mac->make a screen shot without the last line "serial number" and post it here.

Perhaps the FusionDrive "disk" can easily be replaced with a "normal" disk; and then you can use the normal OS instead of the stupid FusionDrive.

Lex

If you could try reposting the screen shot, that would be helpful. If you didn't before, click the picture icon underneath the reply window, second from the end.