got heating but cant get hot water !!?

the baxi boiler that i have experienced numerous problems with over the years...has yet agin malfunctioned !! this time it is a problem with the hot water... When i switch on the boiler, the heating comes on ..but doesnt give hot water?? please if anyone can help me with how to get the hot water up and running ...then let me know..we have now been over a week without hot water..but plenty of heating.
Thanks x Jnee x