Check the display of microwave. If the display reads "PF id = 27", "PF id = 30" or "Err", this indicates that there is an error and that the display must be reset.
Clear the display screen. Do this by pushing the "Lower Oven Cancel", "Upper Oven Cancel", or "Cancel Off" button (which button appears on your microwave depends on the model).
Push the "Clock Set" button and then the "Start" button to activate the clock resetting function. Use the number pad to punch in the time and then press "Clock Set" and "Start" once again. The microwave's display should now function properly.