Safari won't open on iMac

See if it works as expected in Safe Mode:

Use safe mode to isolate issues with your Mac - Apple Support

Hold the power button until it shuts down. Then restart in Safe Mode and test.

Safari won't open on iMac and won't force quit.

If you can't open Safari then you can't force quit it. Read the following before replying: Writing an effective Apple Support Community Question - Apple Community

Safari will not open and my Mac will not restart or shut down.