Gardtec 370 causing problems?

The system's been working fine for several years, but today we've had several power cuts. Each time the rkp shows 'PC' but the alarm sounds and there's nothing I can do to switch it off. The external alarm sounds for 20 mins and the internal alarm keeps sounding until I disconnect the battery. The keypad won't acced any code I put in, simply shows 'PC'.
Any idea what the problem might be???