Scantronic 9930 9651 MAINS fail?

We've had a dormant one of these in the house for a while, been bleeping the message that it needs a rest and all for a very long time.

So other day thought i'd try do something, input the PIN, and suddenly went back to telling the date and time. thought it was fixed so tried to set it, and once i put in the PIN again, it told me 'MAINS fail'. have no idea what to do. coopers saying you need to be an official expert to receive help (fair enough) and the company that installed it doesn't pick up their phone, been calling for 3 days.

any help? whats this mean and what can i do?