If you are having trouble with the heating or hot water, there may be a fault code on the boiler. These usually appear when the boiler has completely stopped working. Always take a careful note of what the fault code is as this may help with diagnosis later.
Resetting the boiler
Many boilers have a way to reset them. Often this is marked on the boiler case or is described in the manual. Try resetting. If this fails (and providing the boiler is not making a lot of noise) try the same trick you would do with your computer. Turn off at the mains supply which is often found near the boiler or in the airing cupboard, leave it for about 10 seconds, then turn it on again. Again, notice what the boiler is doing as this might be useful later.
If resetting the boiler has not worked
Mr Google often has some very good ideas. Google the make and model of the boiler and the fault code. You may be lucky and someone may have posted a method to solve the problem.