Alarm not working after power cut Veritas R8?

The power was off for 8hrs. For the first 20 mins the internal buzzer sounded then went off on the timer. This was repeated when the power came back on but the alarm reset with the user code. Several hrs later the internal buzzer went off again but very faintly and then stopped. No lights are on on control panel and the buttons are dead. The light on the main panel is not lit. Have checked mains fuse that is ok.
Any ideas on what the fault is and how to fix it.
The alarm is 12 yrs old and has worked without fault previously.