maybe your batt didnt charge in the first place, nd u ran it the whole time purely on batt power. It would do that if u just plugged it in and it didnt charge.
could be the transformer that has kicked the bucket which would lead to the original battery also going flat? if you can get a multimeter, you should read something like 13v on the battery when the mains power is on and the battery is charging.
Does it close again or is it running out of power? Could be that the battery isn't charging, check all the wires. Alternatively you may need to run a setup, the procedure in on Centurion's website and relatively easy to do.