Optima XM Kepypad Locked Out?

The problem is as above. The story behind it is as follows:

Been doing a lot of renovation / electrical work around the house and switched the alarm off prior to this. (No alarm cables have been disturbed or damaged). Also opened the Control Panel and disconnected the battery. System left like this for several months.

Connected it back up today, the alarm sounded and I was able to enter the code to silence it. Now the keypad is dead, no beeps etc. Only lights showing are Power and Tamper.

I think the problem may be down to the battery being dead, if I purchase another one will I be able to reset the system or is there a more sinister fault?

Many thanks.