If the leaves are browning and shrivelling, going crispy, most likely shortage of water at some point, if not now, but water thoroughly anyway. If whole stems are suddenly wilting without browning initially, that's clematis wilt, and those stems should be cut out at ground level immediately.

I would say the same, give it a light prune and lots n lots of water. :)

Don't prune if its in flower, you'll cut the flowers off, and pruning is dependent on the variety of clematis you have - some need cutting to the ground, some should not be pruned at all.