Dead Imac? Hard drive?

If your Mac starts up to a question mark

A folder with a flashing question mark means that your startup disk is no longer available or doesn't contain a working Mac operating system.

If the question mark appears only momentarily before your Mac starts up normally:

1. Reset NVRAM.
2. Make sure that your preferred startup disk is selected in Startup Disk preferences.

If the question mark persists, follow these steps:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Turn on your Mac and immediately press and hold Command (⌘) and R to start up from macOS Recovery.
3. While in macOS Recovery, use Disk Utility to repair your startup disk.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.
5. If you still need help, please contact Apple Support.

Try booting into Internet Recovery Mode instead using Command + Option + R (assuming this is an Intel Mac).

Try running the Apple Diagnostics to see if any hardware issues are detected. Unfortunately the diagnostics don't detect many types of drive failures.

What is the exact model of this iMac? You can get this information by entering your serial number here:

Check Your Service and Support Coverage - Apple Support

What happens when you attempt to access Internet Recovery Mode using the command I suggested? It is best to have the iMac connected directly to your router using an Ethernet cable in order to bypass any hard to troubleshoot WiFi issues and for a faster more reliable connection.

I suspect you have a failing hard drive. Using Internet Recovery Mode (Command + Option + R) will bypass the internal drive, but some drive failures can also interfere when booting from external media including Internet Recovery Mode.

It would help to know the exact model of the iMac. Do you know whether your iMac uses a hard drive or an SSD, or perhaps a combination of both as a Fusion Drive? Did the diagnostics pass or fail? Keep in mind we cannot see what is happening or not happening, so details are very important for providing anything more than the general suggestions we've presented here.