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

Stuart Haughey
January 2010