songs won't play on iMac with iTunes

If you're still running High Sierra as indicated by your initial post try reinstalling iTunes.

• iTunes12.8.3.1.dmg (2021-04-13) Current build for macOS 10.13 High Sierra

If you're running a later version of macOS then reinstall the OS. Your data should be preserved, but you do also have that backed up, don't you?

tt2

Reboot and try again. It seems in some cases the music subsystem doesn't wake up properly when the system has been in sleep mode.

tt2