i had the same prob.....you have a frozen water outlet. the plastic pipe under the boiler that leads out side is the problem.
The plastic pipe that is outside that goes to the drain pipe, you need to defrost - its cos we have had mega cold Jan 10 and we aint used to it!!.
Should take you 5 mins - tap the pipe that is on the outside wall with a stick all the way along. Pull the pipe out of the drain pipe, not from the wall. Some ice may fall out which is good if not dont worry. next pour hot kettle water over the length of the pipe, the ice in the pipe should shoot out and then water come out. the system will be clear now, and you can reset the boiler and hey presto!!
Neil cole, January 2010