If an ecosystem lost its producers, there would be no way for energy to move up the web and the ecosystem would most likely fail. Without these energy-capturing organisms, herbivores would get no energy or food. Without them, carnivores would get nothing as well. Because decomposers get energy from all three, they would also die out.
If only decomposers were removed, there would be a lack of nutrients in the soil for plants. This would result in the above (loss of energy, food, and other commodities).
I hope I helped!