my back boiler has stopped working for no reason?

my back boiler has stopped working for no reason. I cant get in to see what make and model it is though so it might b hard to diagnose. Its a council fitted back boiler with a potterton house warmer fire on the front. It has a switch for the boiler on the side of the fire and a slider switch for temp control and an indicator light that goes either red or green. Normaly its green but now when i turn the heating on with the timer it goes straight to red light. Also when doing this the mid position valve only opens part way. This used to move with the position of the slider on the fire. The boiler doesnt even try and fire up when it is switched on with the timer. Can Any one help me please? Let me know if more info is needed. Cheers