The most common cause of a crystal failure is mechanical shock. Remember that there are two or more crystals in each receiver although only one is easily replaceable (the other is permanently soldered into the circuit). Either can be damaged in a hard landing or crash as they are just a piece of quartz ground to size and suspended between two post or wires inside the metal can they are usually enclosed in. The crystal can fracture or one of the leads can be cracked and open up under vibration (engine running). If replacing the plug in crystal does not fix the problem, you could still have a broken crystal (the other one) causing your problem.