I've seen this before and it's gonna be one of 2 things
1) if you have access to a programming kit, try a quick reprogram, it's rare but this may work !
2) it's gonna need a professional repair..
I wouldn't normally suggest a person abuse thier equipment but try giving it a few hard knocks face down on a soft surface .. ie. a nice soft but firm chair seat .. if it starts working again you have a bad solder joint in there somewhere, it'll normally take a microscope to find it, therefore i suggest a professional repair.
If the banging on a chari doesn't work, it may still be a bad solder joint, just not one which will reveal it self easily..
i thought of a 3rd thing
The other thing is it may have a Short circuit in there if you recently had a call out and got the pager wet, those things can get water in them if they get a good splash on them, if you think thats possible .. you can open it and dry it out with a hairdryer... or leave it in the sun on an inside window ledge.
Have fun !