Boiler presusure?

The pressure on the boiler (baxi combi 105e) keeps going into the red currently at 4 normal operating should be 0.5 to 2.5. I took the front off last night and there was a red pressure button that you can turn and that released the pressure but i must have held it too long as the boiler went off in the night due to no pressure. Should the boiler be turned off when i try to adjust the pressure, is this a common problem and how should i stop it from happening? Last Christmas we had a new pump fitted could this be the cause?