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How do I fix my 2010 21.5" iMac screen that glitches out and won't allow me to login?  



The screen on my 2010, 21.5" iMac started glitching, and it won't even load the login screen anymore. I just see weird stripes over the Apple logo as it boots up. 

It loads for a while, displays 2 types of weird glitchy patterns (see images)

photo 2020 04 03 19 46 42
photo 2020 04 03 19 46 38

, and then it just makes the start up noise again and restarts the process all over.

Please help!

Try booting into Safe mode
This will not load any 3rd party additions, it will load some more conservative Apple drivers (may cause screen flicker), and it will clear some kernel caches (a cache is saved data in a form that can speed up a program, but is totally redundant to the original source, and thus can be safely cleared). Booting into Safe mode is just an experiment, but can frequently eliminate any 3rd party interference, or a cached item out-of-sync with the world. (Verify Safe mode via Applications -> Utilities -> System Information -> Software -> Boot Mode -> Safe vs Normal)
You should also try an SMC reset and an NVRAM reset
Reset the System Management Controller (SMC) on your Mac - Apple Support
How to reset NVRAM on your Mac - Apple Support

@datafornothinandbitsforfree - Thanks for the suggestions!

I tried the 3 different reset methods you suggested, but unfortunately I'm still getting the same spinning beachball and glitchy screens. It never lets me get to the login screen. I think I'll probably just have to wait until this lockdown situation is over and take it some place where they can get inside the computer and see what's wrong with the monitor.

I appreciate your help though.