iMac won't start up?

There are many ways to fix this. I will list the methods you should do in order to help.

Method 1: Boot in Safe Mode

1. Shut down your computer completely

2. Wait 10 seconds

3. Boot up your computer and press and hold shift as soon as you hear the chime

4. You can release the shift button when you hear the chime again.

5. Your problem should be fixed, and if there are on-screen instructions, then just follow them.

Method 2: Use Internet Recovery

1. Shut down your computer completely

2. Wait 10 seconds

3. Boot up your computer and the and hold Option while booting

4. Release when you see a few item pop up on the screen.

5. Select a Network and press the arrow button

6. Your problem should be solved, and if there are on-screen instructions, follow them until it is booted up

Method 3: Resetting the NVRAM

1. NVRAM (Non-Volatile Random Access Memory) are sometimes responsible for random bugs.

2. Shut down your computer completely

3. Wait 10 seconds

4. Boot up and Press Command-Option-P-R

5. When the proper screen pops up, release and follow instructions (Wait for second chime to release)

Method 4: Recovery Mode

1. If none else works, reset your computer, and later you can restore your backed up info.

2. Shut down your computer completely

3. Hold down Command-R while booting

4. Follow on-screen instructions to reset.

Hope this helped you out!

-J

If you want to preserve the data on the startup drive, and it's not already backed up, you must try to back up now, before you do anything else. It may or may not be possible.

There are several ways to back up a Mac that is not fully functional. You need an external hard drive or other storage device to hold the data.

1. Start up from the Recovery partition, from Internet Recovery, or from a local Time Machine backup volume (option key at startup.) Launch Disk Utility and follow the instructions in this support article, under “Instructions for backing up to an external hard disk via Disk Utility.” The article refers to starting up from a DVD, but the procedure in Recovery mode is the same. You don't need a DVD if you're running OS X 10.7 or later.

If you use FileVault 2, then you must first unlock the startup volume. Select its icon ("Macintosh HD," unless you gave it a different name.) It will be nested below another disk icon, usually with the same name. Click the Unlock button in the toolbar. Enter your login password when prompted.

2. If Method 1 fails because of disk errors, then you may be able to salvage some of your files by copying them in the Finder. If you already have an external drive with OS X installed, start up from it. Otherwise, if you have Internet access, follow the instructions on this page to prepare the external drive and install OS X on it. You'll use the Recovery installer, rather than downloading it from the App Store.

3. If you have access to a working Mac, and both it and the non-working Mac have FireWire or Thunderbolt ports, start the non-working Mac in target disk mode. Use the working Mac to copy the data to another drive. This technique won't work with USB, Ethernet, Wi-Fi, or Bluetooth.

4. If the internal drive of the non-working Mac is user-replaceable, remove it and mount it in an external enclosure or drive dock. Use another Mac to copy the data.

Apple is very clear about how to handle a Gray Screen on startup. Please carefully read this document and then very methodically go through each and every step until the computer starts up normally. If it does not startup normally, this indicates there is a hardware problem that Apple addresses in the Learn More section of the article.

Please ignore methods # 2 and 4 by Shadowisper as they are incorrect.

2. Internet Recovery requires Command + Option + R to connect you to Apple's servers. It will require that you erase your drive and then it will install the original OS version (if that was Lion or later).

4. Recovery mode will reinstall your current OS version on top of what you have - it is not a "reset".

Slight correction on # 1: when booting into Safe Mode, you should use your computer to see if the problem(s) exists at that point or not as it is a method to help you diagnose your problem:

OS X Yosemite: Start up in safe mode