What will happen if we kill all the organisms in one trophic level?

If all the organisms in one trophic level are killed then the transfer of energy to the next trophic level will stop because of which the organisms at next trophic level will die gradually due to unavailability of food and energy. Also it will cause over population of organisms at previous trophic level because their predators (consumers) will be killed. Thus, it will disturb the food chain completely. For example- In a food chain = grass → deer → lions.

If all of the deer population is removed, no energy and food will be transferred to lion population (lion is carnivorous i.e. it obtains energy by eating flesh of other animals), as a result the lion population will either gradually die due to starvation or move into city and attack humans. Also, due to death of all deer population, the grass vegetation will increase because there will be no deer to eat them. The entire food chain and hence the ecosystem will be disturbed.