Take the cover off the alarm box outside your house and look for a tamper switch. Check that the spring is still attached to the switch, if it is, check the operation of the switch by simulating the cover being replaced by pressing the spring/switch.Sometimes the spring loses its springeyness, so stretch it out slightly and try again. I am assuming that you have checked for loose or broken wires.....P S use ear plugs whilst doing this.
Scottydog
April 2010