Alarm was installed by previous owner . Now it goes off on its own. How to reset the alarm or disengage it?

Sunshineblues, October 2006

Hi,
My car alarm has turned itself of and the red light is always on it doesnt even make a noise when you open or shut it with the remote please can you tell me how to reset it? It worked the other day but when the car door was open i tried to lock it with the remote and pressing buttons and it has turned itself of?

Luke Radcliffe, August 2008