I updated my iMac to Monterey and all was okay, today there was a security update and when my iMac restarted was on High Sierra and a desktop with app icon and files that I had in 2015 I think. What I can do now? It's absurd!
Go into the Startup Disk System Preferences to see if you have more than one bootable macOS volume to select. Maybe you are dual booting this iMac without realizing it.
Go into the Startup Disk System Preferences to see if you have more than one bootable macOS volume to select. Maybe you are dual booting this iMac without realizing it.
Is there any third party antivirus, cleaning or any other maintenance apps installed?
iMac 2013 updated to Monterey after a today's security update restarted to High Sierra
