Unable to boot my iMac

Spouse's iMac computer fails with error message: Your computer restarted because of a problem. Press any key or wait a few seconds to continue startup.

Problem: It NEVER starts up. I tried to start in SAFE mode but that did not work: iMac tried to restart 5 times getting to about 50% on the status bar before restarting again. This message occurs on the second pass of a traditional startup.

Alas, "content-free" problem description: iMAC was purchased new in 2020. No idea of which OS, other system info since of course it doesn't boot to give me that info. He does not recall which background logo. Has been fully backed up. Computer was turned off during a vacation, and now won't boot. Can't run any diagnostics obviously.

Ideas? Drag to Genius Bar?

[Re-Titled by Moderator]