Mouse and keyboard not responding on iMac after upgrade

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 Start up your Mac in safe mode - 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. 

Restart in Safe Mode using wired keyboard and mouse. Continue holding down the Shift key until it restarts fully, this can take up to 5 minutes. Remember a 2013 iMac is very very old and it well past its service life. Consider replacing it.