If you are looking at your boiler & the gauge is low i.e. under 1 bar, there are a few simple steps you can try to top up the pressure in order to resolve the problem yourself.
Locate the filling loop. This can either be somewhere under the boiler on the pipework, or in the airing cupboard near the cylinder, or be built into the boiler itself. (Some boilers come with their own key for filling up). Some filling loops have a lever on each side & some just have one. If you have two levers, turn one so it is inline with the pipe & then turn the other slowly to the same position as the first watching the needle on gauge increase as it fills up with water. (If you only have one lever, then it’s one less lever to turn, it’s that simple). Once pressure goes just over 1bar stop this by turning the levers off to their original position. Please note, do not leave the levers on as this will overfill the boiler.
If your pressure continues to drop on a regular basis, this could indicate you have a further issue which needs to be looked into by a qualified boiler engineer.
See images for examples of filling loops on boiler and on pipework.