When I push the knob for the dryer to start.

If a dryer is not working at all that's not good! But it doesn't mean you need to go appliance shopping just yet. It may be something very simple like a broken door latch, a bad door switch or even a broken door switch actuator, in a kenmore dryer there is a tab thats sticks out of a hole in the wall that allows the dryer to run only if the door is shut, other models may have button that protrudes out of the dryer and when the door is shut, this allows the dryer to run. These switches go out from time to time, also if a dryer is full of lint build-up over time this will cause the dryer high limit or fuse to blow. The fuse is located near where the blower is in most cases. When the air in the dryer gets way too hot the fuse/hi-limit will blow and the house wont burn down.