had power failure now alarm not working?

neighbour cut electric power to my house while working on his drive. i had no electric for 5 hours. my alarm was on at the time, when i returned home my panel on the scantronic series 3200, (modern alarms system) said 'lb' call engineer. The manual book showed that lb ment low battery. so as the alarm is 18 years old we thought the battery back up must have died on us, as we have never changed this external backup in all those years. so i ordered a new battery backup, thinking that all i had to do was replace the battery and the problem would be sorted. but no, when we replaced the battery all we got then was a blank panel, with no lights on, not even the power light or the fault light, which were both on before we changed the battery. we cannot set the alarm or do anything to get it to work. i don't want to have to pay out for an engineer. we have checked the fuses they are all ok. is there anyone who can please give us advise. thank you