Not enough info.
Is it a fanned flue 240 or a balanced flue?
If you check the flue outside it will either have a small 4inch round pipe (Fanned flue) or a large 10-12 inch metal box (balanced flue).
If fanned flue you might try re-setting the over-heat stat. It's a small red button on top of the main control panel.
You will need to remove the white front cover (just lift and pull forward).
You should then see the reset button on the metal panel just above the controls. If you press and it does'nt click then it probably had'nt tripped but in the abscence of more info i can't suggest anything else.
If you write back try to put as much info as you can like does it have a pilot light, can you hear the pump running , if it has a fanned flue can you hear the fan outside? Also what pressure is in the gauge on the front? It all helps to point in the right direction of the fault..
T.h.s, June 2007