iMac won’t connect to wireless keyboard

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 How to 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. 

Sometimes it helps to connect your wireless keyboard through a hub to your Mac. It also doesn't harm to try a SMC reset How to reset the SMC of your Mac - Apple Support

Then you you need to borrow one from a neighbor that has a PC or something.

if it's an Apple keyboard check that the keyboard switch is turned on