After 3 week sleep my iMac desk will not acknowledge me entering my password.
Try booting into Safe Mode according to Start up your Mac in safe mode - Apple Support. If successful reboot normally.
NOTE: Safe Mode boot can take up to 10 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
Also with slow iMacs like the 21.5" model it's good practice to reboot frequently, every other day, to clears out system and application swap and temp cache files which will help improve the general performance.