If you have no heat or hot water, the most common reason is that the pressure has dropped on the boiler.  For a  central heating system to work properly the water pressure should remain constant. Modern combination boilers are pressurised systems and water will enter the system from the mains cold water supply through a manual filling device, known as a ‘filling loop’. Adding water via the filling loop to the system after a drop of pressure will help bring your system to the correct pressure again and the pressure gauge will then increase.  This should be at 1.5 bar.  Refer to the operating instruction leaflet for the make and model of the boiler for instructions of how to do this.  If you do not have an instruction booklet, these can often be downloaded from the internet.