why doesn't the keypad work for our alarm?

We have a Veritas alarm, all the sensors are working, on the main box - the buttons are not working, zone 1 (the front door) is showing a fault on it and is red and the the unset box is green. We have changed the fuse and that hasn't worked, but we don't think it is the battery because the box appears to be showing a fault. When we press the buttons they are not making any sound and do not respond therefore we cannot enter our code. any ideas?