Your dryer has a door switch, located on the inner part of the door frame. The door switch will make contact when the dryer door is shut, thus letting power flow down the series of circuits. Trying to start your dryer with the door open and its not going to start.
The door switch often fails for no other reason than use. Most times the actuator will break off, when this happens the door never activates the switch. I have only heard of several instances where an article of clothing has caught the actuator and broke it off. The door switch in normally tucked tightly in the corner of the inner door out of the way.
If the actuator is in place and you want to verify proper functions of the door switch, you will need a multimeter also known as a volt meter. Also you will need to have access to the switch, if you have not already done so access the door switch and remove from the unit. With voltmeter in hand test the door switch for resistance or continuity, in the closed position the door switch should have continuity.
Accessing the door switch varies on different model/serial breaks. On the majority of dryers the door switch is accessed through the top of the unit, the switch could be pressed into the sheet metal or screwed. On front load style dryers the switch will have two tabs depress and push the switch out through the front. On conventional dryers the door switch will be screwed in with two smaller Philips screws.