Why can't I get hot water ?

On a combi boiler you should check that the hot water temperature is turned up on the front, check the isolation valve underneath the boiler on the cold mains pipe is vertical/open. After this it is likely there is a problem within the appliance. On either a heat only boiler or a system boiler if the central heating work and the hot water doesn't this will be a system problem, possibly a zone valve not operating.