problem with thatcham cat1 alarm?

I have a bmw 1996 323 with a thatcham cat 1 alarm on it, The car is in storage at the moment, now and then i have started the car to keep the battery charged but on 2 or 3 occasions this winter the battery has been completly flat so i have taken the battery off and charged it back up. The last time I did this when I put the battery back on the car would not start. The key fob will open and lock the doors but the little red light inside the car that used to flash no longer comes on. When the key is put in the ignition the dash lights come on but the engine will not spin over would much appreciate any info on this problem,