accenta g3 alarm not working unable to set alarm ??

i have an accenta g3 alarm installed, during a power cut the alarm wasnt set as i was at home. the power went off setting off the alarm which lasted a minute or so. then the siren went lower & lower in volume until it completely went off.
after power back on the alarm would accept my code (****) then would not set.

i have already replaced the battery in the box as the siren volume getting lower & lower indicated the battery back-up was no good ( as it only lasted a minute )

normally when i enter the code get a bleeping noise then i open my door , shut it,after a few seconds the alarm is activated.

now i enter my code & nothing happens. all lights are ok. ive got red for power & green for day what it normally is.

can anybody please help its driving me crazy ????