How do I make the hot water come on after the pipes were drained to take off radiators?

We are having the living room decorated at the moment, and the person who is doing it just took the radiators off the wall and drained the pipes so that no water came through whilst he was doing it. However, he didn't put the water back into the pipes when he had finished, and as a result, we can only get cold water out of both taps, which is a big problem as we cannot have any showers. We have a modern combi-boiler, so how can I get the hot water to come back on? Thanks a lot for answering!