If you your boiler pressure drops below 1.5 bars or below what your owner’s manual for your boiler recommends then you need to add water to refill your system.
It is nature for boiler pressure to decrease over time but if you are generally doing it more than once a year then you should call a gas engineer to find the problem.
Below are some videos that cover how to add water to different types of systems to get correct boiler pressure.
If boiler pressure too high then bleed your radiators.
Combination boilers or “combi” boilers have a feature that makes them stop working due to a too low drop in pressure. In most cases this is easily rectified and will take seconds. Pressure can be lost if you have recently bleed your radiators. If your system consistently loses pressure than you have a leak somewhere. This could be something as simple as a bleed valve that is not closed properly or a radiator valve that is leaking slightly, check all radiator pipes and valves for evidence of leaking. Leaks are normally very easy to spot as the water in the system is discoloured, if you find a leaking valve try to re-tighten it to stop the leak, otherwise replace the valve.