i think it has 'locked out'.
1. locate master panel (big white box approx 10" x 12")
2. remove cover and gently pull one wire (red OR black) off the battery.
3. with a torch to hand, turn off the house electrics. ( the alarm is now 'powered down' (When powered up again it will be a bit loud, so don't jump!)
4. after approx 10 - 20 seconds ........switch electric on, reconnect the lead to the battery and refit and secure the cover.
5. enter your 4 digit code, followed by 'reset'. and you should be back in business.