Clematis are prone to 'Clematis wilt', have you completely followed the planting instructions? The root area needs to be covered in stones or gravel to keep the roots shaded, you could try and cut it right down and water well, it may come back.

Not wilt, if the plants die from the top down In wilt they die from the bottom upwards. This is either mollusc damage or lack of water. And sorry, but the old advice of putting stones or gravel over the roots just gives the snails which love to munch on the stems, somewhere to hide.
However, the plant ought to regrow from the base when eaten by snails, unless they eat the new growth as it comes through.
Is your planting in the ground or in pots?

Clematis wilt in large-flowered hybrid cultivars may be caused by the Phoma clematidina.
Wilting in resistant hybrids and species clematis is very unlikely to be P. clematidina, and is probably caused by environmental problems.
Cut the affected stem/stems to the ground and remove, I have found that when young some Clematis may wilt, but it seems to stop as they mature or it may be we get better at planting/caring for them and the climate is right! One of mine had wilt all the way up the stem yesterday I think its been far too cold and wet? I have removed the stem and am expecting a full recovery.

I am never sure whether my Clematis wilt is fungal, or mollusc damage or wind damage! However, I do find that it happens much more with the early large-flowering types than with any others. Nancy, I feel for you...it's very frustrating. And when the plant is covered with buds and the next day....disaster....heartbreak in my life! ;) Anyway, make sure you plant them good and deep and give them lots to drink and plenty of food. Make sure they are well tied in because wind can easily break the vulnerable new growth. Try to protect them from slugs and snails as much as possible (although to be honest, most of my slug and snail damage seems to be to early new growth and the plants do recover from that in time). Finally, consider growing some type three clematis (there are lots to choose from including some with large flowers and all the viticella group) as in my experience, they do not seem to suffer from these problems. Good luck! :))

Thank you for this advice. I have one clematis that I have had for 4years and there has been parts of it wilt 3 times now. Two well established ones were bent back when my garage was raised to set a cement floor. Looking back now I should have cut them back instead. I now have some clematis that have self seeded and are growing in odd places in my garden. One of these has smaller leaves so I am curios to see what it is. I have 3 more in pots , ready to be planted so I will choose my spots with care. I wonder if putting a bit of copper loosely around the stem will discourage slugs?

Definitely worth a try, though I have never used them...or what about a beer trap around particularly precious plants? They do seem to enjoy the odd pint!

Erm, I have to disagree with Owdboggy - Clematis Wilt starts at the shoot tips, so the problem you describe is probably clematis wilt, assuming you're growing hybrid varieties and not species (montana and the like are not susceptible to clematis wilt).
As soon as you see a stem wilt at the top, trace it back to its point of origin and cut it out right at the base. There is no other treatment for this problem.

Whoops, another senior moment. Put it the wrong way round. Wilt top, snails bottom.
Actually the only real way to tell is to cut the stem across and look for a purple stain inside the stem.
If it is Wilt then there is no point in planting a new Clematis in the same place as the pathogen is persistent in the soil.

Oooh, thanks for that tip owdboggy, I never heard that one before! :D

Owdboggy that is very helpful .. I will check my clippings today.

An old gardener once told me he planted his clematis through a piece of drainpipe or similar and that he found this prevented wilt - can't remember why but he had some lovely clems.

This info. is interesting as I just got my first Clem growing. It has a first bud but I fear I don't have enough space for it. I might have to bring it to one of my clients gardens but it's so happy where it is. I hope it stays as healthy as it is as I'm so excited.
So sorry to hear about Clem' wilt tho'