how can i fix no hot water from glow worm hide away boiler?

No hot water but central heating ok from glow worm hidaway open flue boiler. hot water tank is only 6ft from boiler convection heating coil in hot water tank. suspect no water in the hot water heating circuit. if so how is it filled up again. has original water boiled away? central heating works if just the central heating selected on programmer, but as soon as hot water selected the boiler shuts down. can anyone help please.