How would the loss of consumers affect producers?

In an ecosystem, there are producers of food and there are consumers. The consumers can be herbivores that consume plants or carnivores (that predate on other consumers). In the case of a particular food chain, the removal or modification of consumers or producers severely affects the other.

For example, in a grassland, the producers are the plants such as grass, while deer are the consumers and wolves are the carnivores, and in this case secondary consumers, who predate on the deer. If the deer population is lost due to sudden disease or any other factor, there would be no check on the grass quantity (since there are no consumers) and hence the grass will grow uninhibited until no more resources are available for the grass and other plants. If instead of deer we remove wolves from the food chain, deer will have no predators and their population will grow unchecked until they eat all the producers (grass) and then their population will fall drastically. 

Thus, removing the consumers, whether herbivore or carnivore, has serious implications for the producers.

Hope this helps.