This happend to mine today. Apparently something is frozen. To fix you need to take off the outer casing and under the right hand side of the boiler there is a red screw cap. Unscrew this and this will relase some mucky water. Screw back up and turn the heating on and it should work.
As you can see i am by no means a techical person, but a kind gas engineer talked me thorugh the process.
Good luck.
Rachel
January 2010