If you put something in the microwave, close the door, but forget to hit the start button, nothing will happen. The microwave will not cook the food or heat the item until the start button is pressed. This is because the microwave oven only generates microwaves and applies heat when it is in active mode (i.e., when the start button is pressed).
When you press the start button, the microwave's magnetron generates microwaves, which are electromagnetic waves that excite water molecules and other polar molecules in the food. This excitation causes the molecules to vibrate and generate heat, which, in turn, cooks the food.
If you forget to press the start button, the magnetron remains inactive, and there are no microwaves produced to heat the food or item inside the microwave. The microwave oven effectively acts as a closed chamber with no cooking process taking place.
As a result, leaving something in the microwave with the door closed but not hitting the start button will not cook or heat the item. The length of time the item will remain cold will depend on the surrounding room temperature and the insulating properties of the microwave's interior. Generally, the item will remain at room temperature or cool until someone presses the start button and activates the microwave's cooking process.