Accenta G3 Alarm - no power, stopped working?

Hi - My 12 year old accenta G3 has completely stopped working. Was fine at 6pm, set and unset whilst I nipped out. 8pm, alarm goes off for no reason, cannot turn off as no power to remote keypad, and no lights on main box either, or to any sensors .... have checked all fuse boxes/fuses, and looked for problems with cables to no avail. The alarm has just gone off (after an hour) due (I suspect) to the internal battery running out - any idea what could could have caused the problem, is there an internal fuse that could have blown, is this is a common fault, is 12 years just the lifecycle of the unit .... help appreciated ...