First--kill the power to the fan and remove it, dangling from the wires is dangerous. 1) Did the screws come out of the box as "rossfingal" asked? Or- did the mounting box and all come out of the ceiling? IF the screws came out of the box you should check to see if these were the proper sized machine screws, not wood screws as I have seen. If this is the case you may only need proper sized machine screws. But--IF the box came out of the ceiling you have a different problem, most likely improperly installed box. Then we get in to a single story or multiple story home, different ceiling mount boxes for these. May be in your best interest to hire someone, someone highly recommended.