My Ceiling fan stopped spinning. I detached the fan blabes and turned on the fan and the motor spins. What can cause it not to spin with the fan blades attached?

A weak motor capacitor would cause the motor to not develop enough torque to move the weight of the blades.

A simple test (with the blades attached) is to spin the blades in one direction and then turn the fan on. If the fan continues to work, it is most likely the capacitor.

Depending on the age of the fan, it might be simpler to just replace the entire fan.

I already replaced the capacitor. I was wondering if the 3 way speed switch could be faulty? The motor also hums. The fan is 30 yrs. Old!

At 30 years old, the motor might just be unable to start any more. However, are you sure that you replaced the cap with one that was the "same as" the old one?

If you did not get the specs off the old cap, you might have installed a cap that was to small for the drive needed by the motor.

Anyway, I would just replace the fan unless there is some sentimental reason for keeping the fan.