Turned off power and the alarm veritas 8 doesn't stop sounding?

Turned off the power for about 20mins and the alarm hasn't stopped sounding ever since. Tried to turn it off alarm from keypad, but didn't respond at all. In the end we had to disconnect the internal battery in the panel. This never stop the outside bell sounding and eventually bell battery went flat.

Tried to refit the panel battery in case its reset and still had no control from keypad.

Not sure what I can do with this problem?Please help?