Diagnosis
PF is Power Failure. The microwave lost AC power abruptly. It's a memory reset, not a fault.
How to Fix It
Unplug the microwave from the wall outlet for 30 seconds. Plug it back in. The PF display should clear. You will need to reset the clock.
If That Didn't Work
The outlet itself may have a loose connection or the home's circuit breaker tripped. Check the breaker panel. Plug a lamp into the same outlet to test it.
Heads Up
PF can appear after a brief power flicker. It doesn't damage the oven. Consider a small UPS if this happens often.