When the start button is pushed the dryer comes on but won't stay on once it is released. If I hold down the button the dryer will turn but will not heat up, and will stop as soon as I release the button. Wha'ts wrong with it?

Your dryer was made by Whirlpool. It is of the 27" variety. This dryer has an electromagnetic coil in the start switch that basically keeps the switch contacts closed after you release the switch. If that coil is bad, the switch won't stay closed and the dryer will stop. Replacing said switch would then repair the dryer. How long did you keep the dryer running before checking for signs of heat? Could take a minute or two. If it really isn't heating, you may have two problems. Sorry, your dryer may have 2 problems. Its an electric dryer, so the exhaust thermal fuse wouldn't let the dryer run at all if it was open. If the heater box cut off is open, the dryer would run fine, but wouldn't heat. I can't remember off the top of my head if the neutral for the start switch coil comes from the door switch or not, but if it does and the switch is bad, your condition would result. I do know the power for the coil comes from the timer and is passed to the motor by the switch when you push it, so I think its a neutral problem at the start switch. How good at schematics are you? Sorry for all the "outloud thinking", but its all I can do right now. Let me know if you have further questions.