It doesn't work at all
1. No power to the dryer
Make sure there's power getting to the dryer. Check for a tripped circuit breaker or a blown fuse. Check the wall socket for power with a voltmeter or by plugging something else in.
2. Door switch / Door switch actuator lever
The dryer would not start with a broken door switch. Replace the door switch if found defective.
Most dryers have a door switch actuator lever - when you close a dryer door, it presses against the lever, which actuates the door switch. If the lever is broken, the switch would not activate and the dryer would not start. Replace the lever, if broken.
Note: Door switch actuator lever usually comes with the door switch assembly and it is recommended to get the whole assembly since there is almost no price difference between the door switch assembly and just the lever. Check your model for more details.
3. Thermal fuse
Most dryers have a thermal fuse, which burns out when the dryer overheats, in which case the dryer will either not run at all or stop heating. The fuse is usually located on the vent duct, inside the dryer. A blown fuse will show no continuity when measured with a meter. Before replacing the fuse, make sure the blower wheel is not broken or clogged, and there is nothing blocking the venting.
Note: It is recommended by most dryer manufacturers to replace a hi-limit thermostat when replacing a thermal fuse.
Replace if defective (note: uncommon problem).