Yep, a common problem. Remove the caseback carefully. If the screw has come loose you don't want it to fly out because you'll never find it. Work over a dark cloth, that way you'll be able to turn the watch over and see what falls out. If indeed the rotor screw has come loose, simply find the screw and re-attach the rotor. If not, look at the case clamps, they have a habit of coming loose and impeding the movement of the rotor.