Q: If your burn most of your calories when exercising, what will happen? Will the body burn something else?? Will it stop functioning?

A: Your body will always have fat reserves to dip into for energy as well as burning existing organs and tissues (including muscle) for energy. So it's extremely unlikely you'll exercise yourself to death when your body simply has no energy left for basic processes like keeping the heart beating. But in the worst-case scenarios you can starve/overexercise yourself into a severe exhaustion when you literally can barely do anything other than lie in bed all day long.

The more relevant question may be how your body is using its various energy reserves (fat, tissues, organs) to fuel your everyday functioning and exercise. The answer to that is a lot more complicated because your body is always drawing on all of those reserves even at the same time as replenishing/rebuilding them. Your goal is, in general, to influence your body to draw on fat reserves more for energy and to rebuild the tissues and organs more with extra calories. How you accomplish it is through proper diet and exercise, as described in all the Q&As and articles on my site.