I push the start button on my frigidaire dryer it hums but it won't start... I've already changed the belt and the ball barrien and it still won't go... what it the matter with it.

It sounds like the drum is binding due to a bad bearing. Can you rotate the drum by hand by grabbing one of the supports inside the drum and rotating it? If not, you need to remove drum and check rollers that support the drum. One or both are binding and need to be replaced if you cannot rotate it. Another possibility is bad motor bearings which would call for a new motor, but this is very rare. DrTom4444