Hi Hacks, I have had a similar problem with a clematis in my garden. In June it had some lovely wine red flowers and then suddenly the top growth has turned black and the plant looks like its about to collapse. I have read that it could be clematis wilt, and the best thing to do as obelixx says is to cut it down to ground level and wait until the spring to see if it recovers. It is due to a fungal disease which can stike without any warning.
If you are growing it against a wall or fence my gardening book advises to wash the area with a mild disinfectant.
It also suggests that a way to prevent clematis wilt is to plant clematis deeper than usual. Good luck! When the rain stops, I'm off to chop it down. Fingers crossed for the spring!