iMAC 2017 21" can't fully reboot to allow access to Disc-is there a "Force Restart" option?

First I would test after disconnecting any peripherals other the mouse and keyboard to eliminate those variables,

The closest thing to a "force restart" is a Safe Mode boot.

It opens only most important bits of macOS needed to run the computer. It bypasses any aftermarket cr@p that may be causing issues. It can do some disk and file repair and is always a safe option:

Instructions here:

Start up your Mac in safe mode - Apple Support

I recommend letting the computer "soak" in Safe Mode for at leas 20-23 minutes to let it do its housekeeping. Only then see it the computer will start normally.

However, iMac storage can be an issue. What storage do you have? Unless up-ordered with Apple's Fusion drive option or a factory SSD-only option, the drive is suspect. Even Fusion drives in that model can be flakey

We see a high number of mechanical drive issues in 21.5-inch iMacs made between 2012 and 2019. The more robust 3.5-inch hard drives in earlier iMacs were sacrificed at the Altar of Minimalist Design, replaced by slower 2.5-inch laptop-class drives. We have a 2011 iMac 21.5" (SATA 3 7200rpm 3.5-inch drive) whose drive performance (data transfer speed) benchmarks are 30% FASTER than any base-model mech drives ( SATA 3 5400 RPM) in 2012-2019 21.5" iMacs.

If you need to force your Mac to shut down, press and hold its power button until it shuts down. That takes about six seconds.

You can release the power button when the screen goes dark.

Shut down or restart your Mac - Apple Support

Having said that it is unclear to me how forcing a Mac to shut down will do anything to convey access to a disc (optical disc / CD-ROM, etc).