I had the same thing happen last week. If yours is the 2 button black fob as mine is, then you need to remove the "Alfa" badge which is fitted in a recess on the fob (it's only held in with double sided tape). Remove the screw, then gently ease the 2 halves of the fob apart with a screwdriver. It can be done honest. I found that the actual button inside had sheared off of the circuit board but with some delicate soldering i managed to refit it. just be careful when you've separated the 2 halves, as the switch may (as mine did) fall apart.
Graham Barnes, July 2008