Prior to the alarm going dead, you would note that the “Trouble” light is on and beeping. Check the manual for instructions on recalling “Trouble” The keypad may indicate A/C failure, if this is the case, you will need to restore Hydro to the alarm system.
The alarm system works on 12V AC hydro. There is a transformer for the alarm panel either plugged into or wired into an AC receptacle somewhere on your premises. It may be at your fuse panel / circuit breaker panel.
If, for some reason, there is no AC going to the circuit for the alarm, the panel will run on a backup battery for several hours. When that battery is drained, the system will become dead. You need to check your fuses or circuit breakers to see if a fuse has blown or a circuit has tripped. You can turn off and on each circuit breaker to see if power returns. Check to see if there is power coming from the receptacle where the alarm transformer is plugged in.
If you are able to restore power to a totally depowered alarm system, the siren will sound for a second or two and then stop. You should see lights on your keypad but the “trouble” light will remain on until the panel battery is recharged. This may take a few hours.