Process of elimination:
Can you feel air blowing out of the exhaust vent when the dryer is attempted to be started in the normal way: If YES, most likely the belt is broken.
If NO, Can you hear a 'hum'? If YES, Switch off the dryer immediately. The capacitor that starts the motor is blown. Letting it continue to run will burn the motor out.
If NO, first check the fuse in the plug by putting it into the plug of a working floor lamp or similar. If the fuse was ok, then the motor is now RIP (most likely). If the fuse is blown, the motor could still be blown, you'll find out when the fuse is replaced, it will blow again!
If the fuse was OK and the unit been moved around recently, there is a chance that a connector inside could have come off. Switch off and unplug before doing anything further.
If your nose works above average, if anything is burnt out, you should be able to smell it even if you can't see it (this is not a technical solution).
The door safety switch could be faulty, access it and test it if you know how to. Any other faults beyond this, would probably require taking the dryer apart 'professionally' even if done diy.
Aj
October 2009