backup battery has died
replace the battery is my first suggestion, 12v 2.3ah will fit and be fine for most systems using the Veritas panel
first things first though, remove the mains from the panel using the fused spur, remove the backup battery inside the control panel (larger white plastic box on the veritas) and apply power again and see if that gets it working before buying a new battery
it's common for lower end panels like this to lock up with a dead battery upon mains failure and have the battery drag the system down