Charged wireless keyboard doesn’t work

The following will require any USB keyboard and mouse. If you don’t have them, borrow or buy them, they are inexpensive!

1.     Connect the USB keyboard and mouse to the computer.

2.     Open System Preferences – Bluetooth

3.     Un-pair the wireless keyboard and mouse.

4.     Turn off Bluetooth

5.     Restart the computer in Safe Mode per the directions in Use safe mode on your Mac - Apple Support

6.     Restart the computer normally.

7.     Open System Preferences – Bluetooth

8.     Turn on Bluetooth

9.     Re-Pair the wireless keyboard and mouse.

10.  Unplug the USB keyboard and mouse and begin testing the wireless devices again. 

Was the iMac unplugged from the wall during this time?

First I would try restarting the computer in SAFE Mode and give it 15-30 minutes to :soak" in Safe Mode so SM can finish all its housekeeping.

Start up your Mac in safe mode - Apple Support

Next I wold try an SMC reset, using the instruction set for an Intel Mac withotuteh T2 security chip:

Reset the SMC of your Mac - Apple Support

Is the mouse similarly affected?

Does System Preferences /Settings > Bluetooth "see" the keyboard?

Is the keyboard turned on?

or, if the older model:

I'm not trying to be flippant or judgmental in asking the switch question.. As it is among the most common cause of questsion here about Apple wireless keyboards not working, I have to ask.