Looks like a battery, there is a screw at the top of the main panel and if you unscrew that and prise the panel lid from the top it'll open up hinged from the bottom, take care though as the lids had a habit of suddenly coming loose and the battery inside would fall (on toes) take the battery out and examine it, is it cracked or split, does it sound like there is sand in it when you shake it, if yes to any of these it's kapoot. I'm guessing when you have a power cut everything inside dies and the bell outside starts to ring (classic sign of a duff battery) take it to an electrical wholesalers and buy a new one, Edmundsons / City Electrical / Globe etc etc.
Don
July 2008