This is a common problem that occurs over time. The combination of human touch and pipe movement when water is flowing through them often causes the pipes to spring loose from their attachment inside the wall. There is no great danger of the pipe bursting, but it can be annoying, sometimes unsightly, and-if the pipe leading to the showerhead shakes when it is being turned on or off-it can be noisy. Often there is little space to work with. Usually not enough to see inside, let alone enough to insert a screwdriver or a hammer. There is, however, a simple solution. Push the showerhead back into the wall as far as it will go. Tape the showerhead against the wall so that it will not move. Fill the hole with foam-in-a-can insulation. (Be careful not to fill the hole in the wall completely because the foam will expand.) Once it has hardened, the showerhead will not move.