Optima XM . ?

I have a Optima XM alarm panel. The mains power supply was off for a while whilst the mains electric meter was changed. During this time the outside alarm and strobe were activated. When the mains power was switched on the panel only showed the mains power light and the panel would not reset.The back-up battery obviously needs replacing. When the panel was opened both the panel and outside bell rang and could not be stopped other than by disconnecting the wires to both bells.