What happened to my tomato plant??

When the top of your tomato plant looks like this:

It's time to start looking for THIS:

Tomato Hornworm
A very large caterpillar with a horn on its tail

Here's another photo to give a size reference:

A hornworm's favorite thing to eat is tomato plant, although they will eat potato, eggplant, pepper ... any member of the Solanaceae (nightshade) family.

Hornworms chew leaves and can quickly and efficiently defoliate a plant. Typically, the feast begins with the leaves at the top. As the very hungry caterpillar matures, it may also chew holes in the fruit.

Physical removal is usually the only necessary management, but first you've got to find the beast. It's not as easy as you might think. These creatures are masters of camouflage. Blending in with the leaves and stems, they often go unnoticed until the damage is done. As you search, you may come across another sign of hornworm presence... relatively large, dark green droppings that have accumulated on leaves.

Now ... if you find a hornworm that looks like this:
Good news! Leave it! Parasitic wasps have laid eggs on it, effectively taking over its body. The wasps are "good guys" in the garden. For more about THAT, check out my post from June 2019:
https://sueisinthegarden.blogspot.com/2019/06/tomato-hornworms-and-little-help-from.html