did your soil freeze?

Sure sounds like damp-off (a well documented disease) and your hot caps created the perfect conditions for it. It is most common with young seedlings but the fungus can also attack older plants in the garden when the conditions are right.

I linked the FAQ on it below and Google 'damp-off tomato plant' gives you pics and more info.

Sorry for your losses but now you know one of the problems with using hot caps.

Dave

Here is a link that might be useful: What is damp-off?

I think one of my 28 plants suffered the same fate about a 3 days after being planted out. Never thought it could strike a seedling this large, but when i dug up the plant i saw the tell-tale brown, rotten stem right up to the soil line.
BTW, my plants are not in a tunnell or under cover of any sort, but it has been an unusually hot and humid spring here.