When your boiler has no pressure, for safety it will shut down.
It is always a worry when you come to switch on your boiler after a long period of not using it and you find that the boiler is indicating that there is no pressure. There is no need to panic and first it is better to understand why you don't have any pressure. Then ascertain what you can do to rectify the problem.
So why does your boiler lose pressure? What causes it? and how can you fix it?
There are a number of reasons why and the first can be quite simply down to lack of use or an amount of water being lost from the system by leaks or minor evaporation or boiling over into the safety valve. Water expands as it heats up and contracts when it cools so this is the requirement for the bladder in an expansion vessel. These can split or degrade and then just like a football it can go down and the system will not pressurise. This can be the first reason for the system showing a lack of pressure but there are reasons. These would include as mentioned water loss or a leak meaning that the pressure would drop and in most cases the boiler would not fire up.
If you have been away for a long period or the property has been vacant for some time then be really careful as it is often the case that the water would be very cold so indicating a low pressure but obviously most systems do not go below 10*C if it is a resided in home. So always top up your pressure slowly and in stages otherwise you will over pressurise and the safety valve will discharge into an overflow.
Furthermore if you have only been using the boiler for hot water on either a combi or a system boiler you may find that the system levels have dropped due to natural evaporation or air that was trapped. Trapped air can give a false reading on a pressure gauge so always make sure that you have bled all the radiators before topping up. You also get natural evaporation as you constantly heat and cool the system during the winter so topping up can be frequent but if it is too frequent then you definitely have a leak or an issue.