I've copy 'n pasted. 
Boiler Power Supply
First check the power supply to your central heating is on. Sounds silly? This happens a lot and because customers panic about their heating problems it gets missed. Maybe it’s a fuse or the main circuit breaker board has tripped and just needs resetting.
Reset Button
Most boilers have safety devices built in, so if these are tripped a simple reset may get your whole system working. The reset button is normally on the front of the boiler. Sometimes it's a dial that needs resetting to zero and turned back on.
Can't find your reset button? Check your boilers manufactures instructions - it will be in there.
Heating Controls
The problem with central heating programmers, timers or room thermostats is that customers often find these very complicated, so check your instructions and make sure that they're all set for operation. Some controls send signals remotely and can stop working, so check these too.
All it takes is one wrong button pressed and no heat or hot water, so a quick battery check can solve this.
Gas supply
Gas supply can cause lots of central heating or boiler breakdowns, especially if you have a payment meter with a card. if you run out of credit, the gas will be switched off. So top-up the credit and re-insert the card. Also, press the gas meter reset button in order to re-establish gas supply to your boiler.
Pressure
System pressure in boilers can run low, causing the boiler to stop working and is especially common with combi gas boilers. There is a pressure gauge or dial with 1-4 bars showing. Your system pressure should be at 1 bar when the system is cold.
If it's lower than 1 bar, most sealed systems won't function due to boiler safety devices.