I don't claim to be an expert, but do grow quite a few clematis plants, and this year, for the first time, I'm also experiencing a fair amount of wilt on some of mine. We have also had a very rainy spring. From the reading I've done, it seems that the best thing to do is cut off the wilted parts of the plant and put them in the trash, not the compost pile. I have been taking one of those anti-bacterial wet wipes that come in a plastic container, and wiping off the blades of the pruner every two or three cuts. So far, the plants I've done this with have gone on to bloom beautifully on the unaffected parts. It's a pain in the neck, but I'd rather do it than see the whole vine die back. Wilt doesn't usually kill the plant, because it doesn't reach down to the roots, but it's hard to take, because it invariably happens just as the plant is covered with buds and about to bloom. Some varieties are more prone to wilting than others.

Nakia626, I'm not sure what you are describing is clematis wilt. Wilt starts at the top of the plant and progresses downward...sometimes bottom foliage is unaffected. It never starts with brown, crispy foliage at the base of the plant and a gradual progression upwards.

But there are other problems that could create the symptoms you describe. Unfortunately, a few that come to mind immediately are not quite as forgiving as wilt tends to be. If you are absolutely certain there has been no damage or breakage to the lower stem, the symptoms you describe align very closely with a root rot and it is possible that excessive rainfall combined with less than perfect drainage may have contributed to this condition.

Personally, I know what I'd do if presented with this situation -- I'd dig up the plant and examine the roots. They should be a pale color (creamy or light yellow) and firm and healthy looking. If dark in color, mushy or come away easily, they are dead or dying. I'd also examine the planting hole and see what condition the soil looks - it should be moist but NOT wet.

Or you can make an assumption it is wilt and just cut the plant back and see what happens.