Clematis wilt - only affects varieties, not the species (so not Montana, tangutica, etc) and when it happens, what you see is a shoot that wilts from the top downwards, with the top leaning over a bit like a shepherd's crook, rapidly followed by the whole stem. It doesn't go brown till its actually dead. It can affect just one shoot, or all shoots, but any shoots so affected should be removed at the base immediately. So long as any wilted shoots are removed quickly, the plant should grow away quite well the following year, it only kills the roots if all shoots die back in this way and are left in situ.