I see three possibilties ;
The starting capacitor is shot
The bearings are dry <==== Most likely of the three
The windings are shorted
Shut the juice of to the whole unit, you will find a master shut off connected to the whip (the power cable supplying the juice).
Unscrew the phillips screws holding the top fan gaurd on, remove the fan gaurd,apply a few drops of 3 in 1 oil to the top fan bearing under the fan, do the same to the bottom bearings.
If it turns out to be either the cap or windings then you will have to call a repair tech to have it fixed.