Not trying to be too direct here, but I think the answer is right in front of you. Do a search for "wilt tomato" and you will see a bunch of information there. Tomatillos and Tomatoes are different plants, but are part of the same family of plants so they tend to share similar ailments as far as bacterial and fungal diseases - so what you find may be, at least in-part, applicable.

Think maybe a gopher? I just lost a huge tomato that I had let 'go wild' in an undevelped part of the yard. It was huge, sprawled out, with tons of tomatoes on it. One day healthy, next day dead. Reach under the plant to where the stem goes into the ground. If it is lose and comes up easily, your plant was eaten by a gopher and he is eyeing the other nearby plants. AAARGH!