2000 DISCO II Alarm goes off for no reason

I've had this for a little over a month without any problems. And the alarm has gone off and the remote will not re-set it. Opened bonet and it stopped. Closed bonet and it started. I re-started the engine and it stopped for about 30 minutes then started again. I have used the key to lock and unlock the car, but it still went off. It's kind of anoying when this happens at 3:00am. Thoughts ?