If the battery is running low on the Dell KM636 keyboard you will experience issues with keys not functioning. The power LED will appear as AMBER when this is the case. This can impact any keys but more frequent on C, E or Esc keys.
Power LED is at the rear side of the keyboard - image below
This is due to weak battery power and the solution is to simply replace with a new battery. After changin the battery you will find that the keyboard works normally. Again the power LED will appear AMBER also to indicate this action is required.