how can i fix my vaillant combi boiler?

hello how can i fix my vaillant combi boiler as when i put the heating on the boiler fires up straight away but not for the hot water it did a couple of times when i turned on the hot tap downstairs but it doesnt even do that now and the flow seems slow too even though the gauge shows just under 3 bar any help would be great as i have no money and im getting dirtier by the day!!!!!!!