Are you sure you do not have an airlock?
This is where you have air in a high point in the pipe preventing it for working. If so, the best way is to run a hose from the hot to tap to a cold tap running from the rising main (usually the cold tap in the kitchen. If you then open both taps, the mains pressure will force the air back until it escapes up the vent back to the cold water tank in the loft. You can then turn off both taps, disconnect the hose, open the hot tap and it should run.
Platinum
April 2007