Carbon dioxide is everywhere, so it would enter your body. After a long enough time, all the oxygen in you would be gone. Your body is full of bacteria that help you digest food. Once you die, those same bacteria will start digesting you too. Insects/animals/microbes will start eating what is left. Natural chemical reactions that are normally not allowed when you are alive will break things down to simple molecules. It sounds really gross, but that is how life recycles and without it there would be no life on earth, so it is a good thing too.
I don't know what happens to us after we die. Maybe we continue living in a parallel universe and never even know we actually died. If there is nothing, at least our bodies will be still living in a way as bacteria, plants and other living organisms reuse our bodies and keep the cycle of life going.
