how do i get my pressure up on my ideal isar combi boiler,to get my heating?

the pressure gauge has dropped on my combi boiler, i have connected the lead onto the taps and opened the valve's to get the preesure up, but it does not work.. I can still get hot water although not as hot as normal, but i have no heating... the boiler is on and the thermostate is on high but the boiler is not kicking in..