Hot water not running from taps and heating not operating, WHY?

Just bought house and work being carried out. Valliant boiler was showing No water (Bar low) error message F75. Got pressure up by turning screw below almost fired up, but not fully. Reaslied that the hot water is not running to the bath or sink. Central heating has been working and hot water but had new radiator and pipework carried out to the kitchen. System was working after work carried out so dont think it relates to workmanship. Clearly water related but what would cause this and what is the solution PLEASE? A helpless woman when it comes to matters of this nature.