iMac start up green screen
iMac is stuck with green screen on start up
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Please try to restart in Safe Mode per the directions in Use Safe Mode to isolate issues with your Mac and if that does not work then attempt to reset the NVRAM per How to reset NVRAM on your Mac. If that does not work, then it's likely failed hardware (my guess is a failed GPU) and it's likely time for a new Mac.
You have not told us the year of the Mac of what version of Mac OS is installed but that is where I'd start. You can also take it to your local Apple Authorized Service Provider and have them diagnose it but try the suggestions noted above first.