Bluetooth keyboard on new iMac 2020 suddenly is not working, but mouse is working.

A few ideas:

1. Are you sure it is charged?
2. Turn it off and turn it on again
3. Using your mouse, open System Preferences, click Keyboard and click Set Up Bluetooth Keyboard

If all fail, you can always use the USB/Lightning cable to plug the keyboard directly to the computer. This will help you for the time being but, long term, I would have it replaced.

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 https://support.apple.com/en-us/HT201262

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.