If you are in an area that has had excessive rain this spring, and if the soil in that area of the garden does not drain well (who has soil like that anyway?!), the plants may simply be unhappy because of their wet feet. If the soil drains too well, they may be nutrient depleted as the water drains through the soil.
Clematis are also known for getting clematis wilt. It is a fungus I believe. Maybe you should post some pics of the clem on the clematis forum, someone there may be able to id more possessively if that is a fungus you are dealing with. I have had it before on mine. They promptly turn brown and die. Well I say die, but the tops died off, then it regrew again. So even if you think the clem is dead at some point, don't be too hasty to toss it, because it may not be. I would probably spray with a fungicide as a precaution and the fertilize the area.
