Dryers have a drive belt that turns the clothes drum. If the belt breaks, the drum stops rotating. Then you need to replace the belt, and often the idler pulley, because the pulley tends to wear out at the same time.
Click here for a belt installation diagram
If the dryer motor only hums when you press the Start button, the motor may be burned out. Here's how to test the dryer motor:
Remove the belt.
Check for obstructions in the blower fan housing.
Manually rotate the shaft of the motor.
If the motor is very stiff, or impossible to rotate manually, and the blower fan housing is clear, you need to replace the motor.
If the motor rotates freely, run the motor momentarily with the belt removed and the blower in place. If the motor runs fine with the belt removed, there may be a problem with the idler pulley or the clothes drum. Try to rotate the drum by hand. If it is very difficult to move, correct any problem with the rollers or pulley, and then reassemble the dryer and try it again.
If the motor hums but doesn't rotate on its own even with the belt removed--yet you can turn it freely by hand--you usually need to replace the motor or the motor start capacitor.
Your dryer can't operate at all if the door switch is defective. It's inside the dryer main housing near the door frame. Sometimes you need to raise or open the top or front of the dryer to reach the switch. If it's defective, you need to replace it. To determine if the door switch has failed, watch the part testing video at the bottom of this page.