There is not a certain reason a boiler might breakdown. It could be a variety of things that could cause it to screech to a halt. When this happens you might be left with no heat or any hot water. Instead of panicking and reaching for the phone take a minute and see if the problem is something you can fix.
Many times the fix is something as simple as flipping a switch. Sometimes a switch can be tripped off by mistake. Make sure all the switches are turned to the on position for the electricity, water and even the gas. If one is flipped off then simply turn it to on and see if the boiler comes back on.
Sometimes a thermostat can go bad over time. If the thermostat is not wireless then kick it on up and see if the boiler fires up. If the thermostat is wireless then turn it way down for a few minutes then crank it back up and see what happens. If the power has been tripped or the battery has died then the thermostat will also appear to be bad, so check this also.
When it gets extremely cold outside there is the possibility of the pipes freezing because of the condensation build up. If the pipe is frozen then wrap it with a heat wrap or even a bottle full of hot water. This might have to be done several times before it thaws out.
When doing heating repairs Crystal Palace there could be air that has built up and needs released. The easiest way for releasing this pent up air is by bleeding each radiator. This will get the water flowing properly once again. If your problem is not fixed then it is time to call a professional.