Your boiler must be at the right pressure level for your heating and hot water to work properly. If your boiler loses pressure or the pressure becomes too high, this can cause the boiler to shut down.
The most common cause for high pressure is too much water in the system. To fix this you will need to drain some water from the system. This can be done by bleeding the radiators using a radiator key. Before starting, make sure you have a small container to hand to catch any water. Slowly twist the valve anti clockwise and you should hear air escaping followed by water leaking out. Allow a small amount of water to come out into the container and then tighten the valve again. You will need to check the pressure after each radiator has been bled to see if there is a change.
The other main cause of high pressure is a faulty pressure release valve (PRV). This is a copper pipe which you will find sticking out of the outside wall behind your boiler. If this is constantly dripping then you should contact a Gas Safe engineer. who will be able to replace it for you.