Hi Nancy, thanks for the question.

Looking at the picture it appears this probably started at the bottom of the plant and a perhaps one stem.  This suggests that this may be a fusarium root rot issue.  That may indicate a drainage or overwatering situation.  Verticillium wilt is another possibility as well as other wilt diseases.  Soggy soil can keep oxygen from getting to the roots.  If you have a nearby black walnut tree that be a factor. Check out the following links.

Possible Causes of Sudden Wilt and Death in Tomatoes | Nebraska Extension in Lancaster County (unl.edu)

Fusarium Wilt of Tomatoes in a Home Garden | University of Maryland Extension (umd.edu)

Wilt Symptoms | Cornell Vegetables

I hope this is helpful.