Going on your description I would normally say " you need to get a pro in", but on this occasion i'am going to instruct you on how to shut the system down.
This is to prevent damaging the panel's electronic circuitry because the backup battery could be faulty.At the moment your panels status is indicating a mains failure, but the noise you describe sounds like a shorted battery.
Firstly locate the fused spur that supplies power to your panel and remove the fuse,
At this stage I would expect the bells to ring, don't panic enter your user code, it may silence it, it may not.
Don't worry if it won't silence let it ring'
Remove the cover from the panel and disconnect the battery leads, red and black wires.If the battery is hot, then that's a good indication it has an internal fault (short).
The internal bells will of stopped ringing at this stage now, but i expect the outside sounder will still be ringing, but as a reduced sound level.Let it ding until it's SAB ( battery) drains.
Now you have two options now:
1 get someone in
2 replace the battery yourself ( like for like) and hope the battery hasn't damaged the panel.
If you are not sure or can't find the fused spur ( not fitted) then get someone in.