I have Kenmore Elite QuietPak9 He4 last year. All of sudden it has stopped working. I don't know what is wrong with it. It won't start.

Have you checked your breakers? Do you have power to the unit? If so, check the thermal fuse at the rear side of the vent housing inside the unit for continuity. If none ,replace. If that is OK check the door switch the sam way. If no continuity there, replace it. Good Luck