If you have hot water but no heating then it could be caused by one of the following:
The first is that there is an issue with the settings on your boiler or central heating. This should be relatively simple to check and adjust if this is the issue. As obvious as it may sound, check that your heating is turned on. Also check that the temperature on your thermostat is turned up high enough. Another thing to check is any timers that you may have on the system. If the clocks have changed or you have had a power cut, then this could be the issue.
If after adjusting the settings, your central heating is still not working, then it could be an issue with a part within the boiler. It may be that the pressure has dropped and your boiler needs repressurising.
If you have any heat at all coming out of any radiators, then the issue is not with your boiler. Try bleeding your radiators to release any trapped air. If your radiators are cold at the sides as well as at the bottom, then you may have sludge in them and require a Power Flush.
The final possibility is a faulty part within the boiler itself. If having checked all these areas you still have hot water but no heating then contact a Gas Safe engineer.