Diagnosis
That's the oven temperature sensor error. The little sensor that checks how hot the oven cavity is has either failed or the wire to it has come loose. The microwave locks up because it can't tell if it's safe to run.
How to Fix It
You'll need to get behind the control panel. Unplug the microwave first—safety is key here. Remove the grille at the top front (it usually snaps off). You'll see screws holding the outer cover over the electronics. Take those off, and you'll see the sensor, a small nub with a wire, clipped into a hole in the oven cavity. Check that the wire connection is snug on the sensor. If it is, the sensor itself is likely bad and needs replacing.
If That Didn't Work
If the sensor connection was fine, the problem might be on the main control board. A bad connection there can cause the same error. You'd need to check the wiring harness that plugs from the sensor circuit into the main board for any damage.
Heads Up
This is not a filter or cleaning issue. The F-3 code is a hard fault. If you're not comfortable working with live electronics, it's time to call a technician. Forcing it to run could be a fire risk.