What is most likely happen to the carbon cycle if there were no more plants on earth?

Plants use carbon to live, just like humans use oxygen. It's a virtuous cycle sine humans breathe oxygen and expel carbon, and plants "breathe" carbon and expel oxygen.

If plants weren't there to "breathe" the carbon, it would just sit in the world, and build up in the atmosphere

The plants give off oxygen we breath oxygen and breath out carbon, plants need carbon to live, the plants would die and there would be a huge lack of oxygen for us to breath