how can i fix?

Gordon - you give insufficient information - presumably you have a conventional boiler with a hot water storage tank and presumably the thermostat you mention is a room thermostat.

If this is the case then it may be that the thermostat on your hot water cylinder is asking for hot water but the mid position valve that directs the water to either radiators - or to the hot water cylinder - or sometimes to both at the same time - it may be that the mid position valve is stuck in the wrong place and is directing the heated water to your radiators.

My mid position valve is in the airing cupboard - with many of them there's a small override leaver - you can even remove the motor control and operate the valve manually if you have the confidence.

Or you could call in a boiler engineer.

Good Luck...
October 2015