Well.....I would first check to see if you have a stem borer, then I would inspect the vine at soil level to see if it has developed crown rot......may even have nematodes attacking the roots further in the soil....... may also be verticillium wilt....
Also.....did you over fertilize (fertilizer burn) or other chemicals accidently applied?
Good luck....

Thanks ians.
Definitely not fertilizer burn.
I'll check for the stem borer & crown rot tomorrow.
I was under the impression that the wilt was more or less associated with Group II clematis.

Clematis are resistant to verticillium wilt (not the same as clematis wilt) and this species clematis is resistant to even that problem. I'd suspect some sort of damage to the base of the vine - possibly borers (although I have never encountered that issue with clematis) but just as likely some sort of mechanical damage, either manmade or critter induced.

Unless in a poorly draining area - and you would have noticed that problem long before now - there is likely no serious problem with the root system. I suggest just cutting it back hard and see what happens next season. IME tanguticas are extremely tough vines - I'd wager it comes back fine.

Thanks gardengal. I was afraid that my spring pruning had affected it.
By hard prune do you mean cutting it to the ground and removing all of the woody stems?

It depends on how large and how woody those stems are. My experience with hard pruning (down to the ground) mature vines - those that have developed a decently sized woody 'trunk' (like 1 inch diameter or more) - is not very favorable.

Personally I would prune back to a framework that encompasses where any lateral branching takes off. There should still be adequate meristematic tissue in these lateral shoots to encourage and support new growth in spring.

Gardengal, hope you get this message.

I did as you said, created a frame work and the plant had new shoots, which were promptly killed by the frost. As of now no sign of revival.

My Paul Fargas, which is further down and in shade is full of leaves but not this one.

Give it some additional time. Since all the buds had formed leaves which were then killed, it may take the plant some time to reform buds and leaf out again. I would be very surprised if it does not recover. Even if the stems were killed by the cold, the roots will still be alive, so you can watch for sprouts from the base of the plant as well as buds on the stems. Sometimes clematis come back after a year or two of not appearing at all.

And that is exactly why I give mine so much time before declaring them dead. I always feel silly waiting a year or two for one to reappear, but it's happened often enough that I do it anyway. After a few years, I'll dig around to see if I can find any roots, and if not, I'll replant in that spot, but more than once, I've dug up a nice root ball, particularly if the clematis was a few years old when it "died".

I lost 'Helios' when a mole tunneled up under it and it's roots were exposed and it dried up at the first hot spell.

There were perennials in front of it, I didn't see the mole hill.

I have had this issue with callunas (healthers) too. Moles don't eat plants but their tunnels can allow them to dry up....

Thanks everyone for your responses.

I actually was so sick and tired of it. I just cut it down to the ground. The thick branches were sort of hollow, and there was no sight of life or greenness any where.

I honestly don't think that I have an animal problem (live in the city). I just find it unusual to have a such a tough clematis die in the middle of flowering. At first I thought it was due to the drastic pruning of it's long woody canes in Spring, but it just didn't make any sense. If it is unhappy why wouldn't it show it right away?

Then I though maybe something is wrong with the soil as the dead nettle (Lamium maculatum) died too. But then the ferns are fine.

Anyway, I'll wait and see.

Pruning is not the problem.

Did someone spray Round Up? or another herbicide? did you spray it with a fungicide or even a fertilizer and not follow directions when mixing?

No, I didn't do anything. However, I thought my neighbour might have done something, but again, if she would have done that, then her plants would've been dead too, which they're not.

It could have been simply because that the combination of a belated reaction to me cutting down it's 6 feet woody branches and then extreme cold finished it off. The Tangutica was planted in a very narrow raised bed.

Or simply maybe it's sulking there and will surprise me in a couple of days or next year...

The combination of extreme cold and a narrow raised bed (which is more likely than either in the ground or a larger raised bed to have issues with freezing and/or freeze/thaw cycles) might have been the problem for your clematis this past winter.

I prune 6' woody Clematis vines almost every year and have never had one die from it. Even those that hadn't been pruned in years or those who aren't supposed to be pruned every year. I have found pruning to overwhelmingly help them rather than hinder them over the last decade.

Babs cold winters, polar vortex style are the norm here, every year. This year was just longer than usual. However, as you say, maybe this year, the cold finished already weakened, with whatever, which ailed it in september.

Thanks buyorsell888 for sharing your experience different from gardengal's above. These were around 0.5 inch thick tentacles, which I cut down.

Thanks all for are your comments and help. If there is any development I'll post an update.