Try reprogramming the fob to the car - it shouldn't have forgotten
anything, but they do on occasion.  The instructions are in the handbook I
think; failing that, I can post them if them I can find the right file ;)

If this doesn't work then a new fob would seem to be the answer.  Cheaper
on eBay than from a dealer, but you need to be careful you are buying the
correct one...

There's a slight chance that you touched both poles of the battery and
it got discharged.

-Nick