There are several possibilities and in all cases the unit must be dismantled to examine whether the fan casing has been damaged or displaced, the motor has been damaged or the internal motor locating lugs of the plastic casing have been broken; the motor is not usually attached to the casing but rests inside the casing, held in place by a number of internal lugs, flanges and other mouldings and the clamping effect of the two halves.
Dropping the unit is likely to damage or break some of the internal detailing of the casing but mostly these things can be rectified by plastic welding and/or the application of propriety adhesives or fillers.