There is a disease aptly named Clematis Wilt that might be responsible. Usually, it can kill the above-ground portions of the plant, but doesn't kill roots, so regrowth might be possible. Clematis also are fairly intolerant of soggy soil or over-watering, so make sure the plants are in well-drained soil and aren't being watered when the soil isn't at least somewhat dry a few inches below the surface. Root rot can create the same symptoms of rapid-onset, all-encompassing wilting. (The current drought is causing many plants to be in need of water, but it can be possible to over-water even during drought, depending on soil qualities and how much, and how often, a plant is being watered.)

For now, all you can do is to cut back all wilted stems, since they won't recover regardless of the exact cause. You can wait to see if regrowth appears in the coming weeks, though the article linked above suggests that recovery might take longer if Clematis Wilt is involved. There is no fungicide that can cure existing infections, and even though they might help prevent new infections, it's not practical to use them in this situation. (Plus, they might risk harm to pollinators.)

Miri