Posted: 14/06/2014 at 16:24
Slugs and snails sometimes scrape the outer part of the stems off near the base and can kill the whole stem this way. The other possibility is that the pot was allowed to dry out in which case it may be dead, unfortunately. All you can do is cut the affected stems off just below soil level and wait. If you are lucky, fresh growth will appear from the roots but be sure to protect this in case molluscs are still around as they'll gobble it up overnight. The good news is that it isn't clematis wilt as that starts at the tips and works it's way down.
Edit: just saw Dove's post - fully agree - they need large, deep and cool pots.