If grass and plants disappeared from the food chain, thefood chain itself shall fall apart. The plants are almost at the bottom of the food chain, right next to the sun. Plants are the fundamental key in the food chain.
Herbivores rely on plants to get food. Now, what if there were no plants? They would all die out. Carnivores would also suffer greatly. Carnivores only eat meat. If there were no more herbivores such as rabbits, what would they eat?
Omnivores would suffer from this too. Omnivores, such as us human beings, eat plants and animals. If all the plants and animals were gone, we would have nothing to eat. The only edible thing left would be water.
If all the plants disappeared, then life on earth would almost be gone entirely. Only organisms such as cells, bacteria, and living things that only need the sun to survive such as algae would survive.
I hope I helped you!