Is the day led lit?,bet it's not.
Is the LCD screen lit?,bet it is.
Is the keypad unresponsive?, bet it is.
Have you ever replaced the +[BATTERY]- ?,bet you have not.
If I'm correct on all accounts then I'm 99% sure you have a buggered control panel backup battery.
The remaining 1% is a blown RKP fuse located on the control panel PCB but they seldom ever blow unless their wiring is cut nailed stapled etc.
On both accounts mains power must be isolated for the control panel BEFORE the lid is removed.
Remember mains voltages can kill.
e manno, December 2013