How do I restart an iMac with a frozen Finder?

If Apple menu > Restart isn't possible, see the following from Shut down or restart your Mac - Apple Support

You can force a shutdown if necessary—for example, if your Mac becomes unresponsive—by pressing and holding the power button until your Mac shuts down. You may lose unsaved changes in open documents.

If you are you able to get to the Apple > Force Quit menu > select Finder > Relaunch

If not, hold the power button for 10 seconds or until the iMac shuts down. Wait 30 seconds or so and press the power button to turn the iMac back on.