Why has my alarm come on and why cant I get it to go off?

I have a BMW 3 series M Reg. I never use the alarm system on the car I just use the key tu open the car. I used the car Wednesday night with no problems but I then went to get into the car Thursday morning but the alarm sounded as I turned the key in the door. I tried to turn the alarm off with the Fob but it doesn't go off. I know the battery in the key fob is ok.

Does anyone have any suggestions what the problem may be as I cant get the car started as now the immobilser is stuck on as well.

Thanks in advance

Alex
March 2010