Most likely cause: Impact to the flash caused the release side flash pivot to fall into the camera body. A small plastic bit on that pivot closes the flash-up switch. This is a well know problem with almost all Nikon DSLRs.
The pivot is hollow and the wires from the flash head go through it, so it is possible to carefully fit a very small hook in the hole and pop the pivot back into position. Make sure you don't break any of the small wires!
There are two screws on the underside of the flash. Remove them and then you can pop off the cover with a fingernail