I have a early iMac 21.5 that sometimes the screen flickers then goes black. Only solution seems to be turning off and on.

It's probably overheating. Make a "Genius" appointment at an Apple Store to have the machine tested.

Back up all data on the internal drive(s) before you hand over your computer to anyone. If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to bare metal from any of them.) Don’t erase the recovery partition, if present.

*An SSD doesn't need to be zeroed.