It sounds as though your plants have been attacked by tomato hornworms. They are large (up to 4 inches long) green caterpillars with white stripes on each side of the body. They get the hornworm name from the black "horn" at the rear of their body. Tomato hornworms may also feed on tomato relatives such as eggplant, pepper, and potato. Just a few tomato hornworms can strip a plant in a short time. The worms often escape detection until much of the foliage is gone, because they blend in so well with the green leaves. If you see hornworms on a plant, simply pick them off by hand (use gloves if you're squeamish), and crush them underfoot. If the hornworm has small white cocoons attached to its body, leave the hornworm alone. The cocoons contain the larvae of a parasitic wasp that is a natural parasite of the hornworm. By the time the cocoons are present, the hornworm is no longer able to feed, so it won't cause any more damage to your tomato. The wasps that emerge from the cocoons will parasitize other hornworms and help protect your tomato plants.