What does sudden red screen mean?

I've relied entirely on Google - "imac disassembly" or try "replace GPU in iMac," for example. I don't know which model you have, but you will need to know that in order to identify what links are going to apply.

Sounds like GPU failure. I would take the computer in for a service diagnostic.

Whenever you are forced to do an abnormal shutdown you should immediately repair the hard drive and permissions to assure there was no damage to the hard drive.

Repair the Hard Drive and Permissions - Lion/Mountain Lion

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Repair

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu. Select Restart from the Apple menu.

If you have the original discs that came with your computer, then there is a diagnostic test you can run:

How to run hardware diagnostics for an Intel Mac

Boot from your original OS X Installer Disc One that came with your computer. After the chime press and hold down the "D" key until the diagnostic screen appears. Run the extended tests for a minimum of two or three hours. If any error messages appear note them down as you will need to report them to the service tech when you take the computer in for repair.

Some "common" error indicators:

SNS - sensor error

MEM - memory error

HDD - hard disk drive error

MOT - fan error

No guarantee it will show anything unless the defect occurs while running the test, so you may need to run the test over a long period of time.

In an iMac there's no "card." The GPU is a chip on the motherboard. Depending upon which specific model the GPU chip may be socketed making it relatively easy to replace once you find it. The latter is the problem as disassembling an iMac is no easy task.

Glad I could help. 🙂