It sounds like you've been the victim of something called 'Clematis wilt'. It's a fungal infection around the base of the plant and will quickly (sometimes within 24 hours) kill the plant. You can help prevent this by taking an old plastic flower pot, cutting the base out of it, then cutting it all the way up the side and placing it upside down around the base of the plant like a kind of collar. - Otherwise, as Cetti said, grow in a sunny position but keep the roots in shade. Stand something in front of it if necessary. Clematis are a very rewarding plant to grow as they produce a wonderful display of blooms, very varied depending on which species you've got. Keep trying, you'll succeed in the end.