How can I re-attach pull switch cord socket to ceiling?

Hi, after pulling the switch cord in the bathroom, the whole socket with screws has come out of the ceiling, revealing that the socket wasn't attached to the beams, but to the plaster board. Now I have two holes in the ceiling where the screws were attached and I want to attach the socket below one of the beams. What's the best way of finding the nearest beam? How do I push the wiring to the new holes I will make in the beams?