How to stop my House alarm going off!!!?

I have a accenta g3 house alarm that was fitted in 2006. The battery was changed a year ago and up until a couple of weeks ago it was working no problem.

Since then the alarm keeps going off when not set, it can go off at random times during the day or night.

Also the door sensors aren't woking, we had sensors on the front and back door as well as the garage doors and now none of them are working!!

Any advice wouild be gratefully recieved as I have been unable to contact our engineer!

Thank you