Peccavi's suggestion is probably correct (and don't mind him - if he thought he was out of his depth, he wouldn't say anything!).
The most common type of electric motor is an induction motor. It needs to create a rotating magnetic field to run. In large industrial machinery, this comes from a 3 phase supply. In small machinery to be run from a domestic supply, there is only one phase available, so this problem is overcome by using a capacitor to produce a 90 degree phase shift in the voltage which results in a rotating magnetic field. Without the capacitor there would not be any/enough torque on the motor shaft which is why it doesn't move. You'll know this type of motor because it has a bulge on the side of the motor housing to accomodate the capacitor.
J
April 2009