Whistling noise and blank screen on my potterton 2001!?

I have a potterton ep2001 controller on my gas heating system.It was working fine when i switched it off but when i switched it on again all i get is a blank screen and a whistling noise from the box.Strangely enough the heating seems to be working even though the light on the thermostat is not illuminated.Any advice would be very welcome!!!! Regards Stuart