What size pots were they in? That can have a significant impact on their ability to tolerate winter cold. I'd also be interested in knowning what type of potting medium you used and if the pots were offered any sort of protection over winter, as in grouping them together, wrapping, heeling into a mulch, moving under cover, etc.

I grow a lot of clematis in containers and haven't had problems with any, including some of the newer Evison introductions, but I am in a much milder climate. Like any other long term containerized planting, one must take into consideration how they will tolerate winters in one's location and what sort of protection one should offer.

You might give it more time. Both my patio clematis got wilt and went underground for awhile. I would keep it watered and maybe it will come back from the roots.

Gardengal, I already mentioned that I have 20+ Clematis in containers of the same size (2G and more) with the same medium (Miracle Gro) at the same spot. All of them are perfectly fine except those two this year and one last year. Even a couple of babies in 1G pots survived and are growing like crazy. All these pots were grouped together and covered with leaves.
To answer the second question: they do not need more time. They are DEAD. Both of them are almost decomposed :-( I can foresee one more question... No, they did not stay in soggy soil and the drainage was excellent.
So, the bottom line: I lost 3 Clematis out of 30+ potted ones after 2 winters. All 3 of them were Evison's Patio Clemmies. All 3 of them had great rootballs when I bought them.

Maybe those Patio clematis have less hardy clematis in it's parentage. I went to clematis on the web to look up the parentage. Picardy's heritage was "withheld". I guess it's a secret. The bourbon had 2 large flowered hybrids as it's parentage. Gardening is a lot of trial and error. I certainly have killed more than my share of plants trying to find varieties that grow well in my climate. At least you now know 30 clematis that will do well in your climate.