Low pressure and no hot water on 24i Worcester Boiler?

We had 2 radiators taken off the wall and put back as we had some work carried out. This did cause problems with the boiler and we had a plumber to replace all the washers in the boiler. We have noticed recently that the pressure is nearly zero, but also we can't get hot water unless the heating is on. Could this be linked? Is there any other reason why we can only get hot water when the heating is on? Any advice would be appreciated :-)