atm

my 2003 mondeo st220 alarm has gone tits up, iv opened the door, started the car, the alarm has started going off and the hazard lights flashing, all while the engines running and it wont stop going off!

iv got out, locked it, the siren has stopped but the hazards are still going, and everytime i unlock the door or open anything the siren starts again! help?

iv only got one key for the car (the non fob key) but its always been fine upto now!