Disconnect the mains, then the internal backup battery, wait approx 30 seconds, reconnect battery,enter your user code to silence the alarm ( caused by tamper as you will have the case open) refit case cover ,reconnect mains. This should fix it for you, do a walk test to verify all is well.
If your battery is over 3 years old, replace it as it is, or soon will be, unserviceable.
Pete, July 2007