It is probably suffering from clematis wilt which is a nasy infection which is difficult to get rid of. You may have to dig it up and not plant a replacement in the same place.
I agree with WendyS, it does sound like clematis wilt, which is most commonly caused by the fungus Ascochyta clematidina.
It may be worth cutting back any affected plant tisue back to healthy tisue as it may sprout new shoots from lower down, possibly below ground, use an anti-fungal paint to cover wounds and thoughly dispose of deseased material.
Good Luck. T bird+