Scantronic 9200 front panel - lights on, keypad not working?

Hi, I have a Scantronic 9200 alarm which was installed in 1986!! I had to switch the mains power off for a while but when I switched it back on, the keypad lights are all on but the keypad is not responding at all. There is a faint noise coming from the alarm outside and the noise coming from the inside bell stopped after 2-3 days. Can someone please advise whether I can reset/fix something (I still have the original manual) or whether it might be a battery replacement job or even something else entirely? The alarm was working perfectly okay prior to this power loss....