Any indication that the stem immediately below this point has been damaged? If not, chances are you have clematis wilt. Relatively common with any number of LFH's and while disappointing, seldom fatal.

Just cut the vine back below any signs of wilt and treat as normal. You should see new growth this season and possibly even some late blooms. But next season it should come back as strong as ever.

FWIW, wilt can be a recurring phenomenon for several years on some pruning group 2 clems but they generally outgrow it in time. Many of the clems I've a grown have been nursery returns because of clematis wilt - I just cut them back and provide some TLC and they are usually as good as new.