How do I fix my non-responsive Gardtec 800 series keypad?

We had a powercut a few weeks ago and the outer wall alarm box has been flashing with a white light ever since. I have tried to reset the system but the keypad does not do anything when I punch in the code - it doesn't even beep when I press the number keys. I have tried turning it off and on again at the fuse box but the problem remains. The digital readout constantly has a 'U' with a peculiar symbol next to it. Any ideas how to stop the white light flashing on the outer box?

Cheers, Simon.