if you look on your bell box outside or on your keypad, it should tell you what company installed your alarm, and ring them and tell them your problem, its easily fixed, its proably just a sensor needs to be twigged or a loose contact,
the company i work for charge, 80 inc vat for a call out, but if you need an extra sensor it would be extra, but there sticklers, they will only fix or service their own alarms, and i know alot of companies are like that because they need engineer codes etc..
there is also a a dialler you can get on your control panel, that you can get that will text your mobile to tell you that your alarm is going off, there is no yearly cost, were as monitoring (garda responce) would cost up to 300 or more a year