Since updating to Monterey the SD card slot on my imac has stopped working.

If all else fails give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

If yours is an Intel iMac, you might try an SMC reset:

How to reset the SMC of your Mac - Apple Support

Are you running the latest 12.4 OS? Earlier Monterey versions had issues.