What would happen to an ecosystem if the decomposers disappeared?

Decomposers, as in bacteria and fungi? If all the decomposers were to die, organic nutrients would no longer get cycled properly; for example, naturally healthy soil is made from the cycle of organic matter. Plants, animals, bacteria and fungus with the help of some erosion break down rock into minerals. They also die, and their bodily materials end up mixed in with the rock bits. This mixture is further broken down and decomposed by bacteria, fungus, worms and other bugs that live in it. The final product is rich in easily absorb-able nutrients, is properly aerated by micro-organisms so nutrients can flow properly, and is really a living thing. It supports biodiversity of bacteria, fungus and insects that keep it cycling organic matter functionally. Without all this, organic matter would eventually seize to exist.

Fungi are key decomposers in an ecosystem. As such, they are key recyclers of carbon compounds in the ecosystem.

Then there would be no plants that will grow in the ecosystem. then the ecosystem will die out or fall apart.

Matter will not be able to be broken down and a lot of things will pile up

the cycle wouldn;t start anymore