Dryer will not start

My whirlpool duet dryer comes on, I can make my selections, the door is shut securely and the light goes out when the door is shut however when I press start it beeps and does not start. It did this probably a year ago and I had a repairman come out. He checked everything and could not figure out the problem here so he scheduled to pick it up and take it into his shop, however before they could come pick it up I just tried it the next day and it started up and has worked fine until now. What could this possibly be?