Clematis wilt is a defined issue with certain types of clematis, primarily those lumped as pruning group 2 or the large flowered hybrids, Cezanne is included in this group.

But the affliction itself can be confusing. While it is often considered to be a fungal issue (Calophoma clematidina), that particular cause is actually quite rare. Most often it is an affliction of younger vines that just grow faster than their root system can support. This can be exacerbated by growing in a container (also by improper planting techniques) that limits free and easy root development. This theory is supported by the fact that vines established for at east 5 years are seldom ever bothered by wilt....it is only the younger, immature or not well established plants that succumb.

The good news is that neither cause is considered fatal and cutting the vine back hard and continuing to tend as a viable plant (proper watering and fertilizing, etc.) should encourage regrowth. It may not be apparent immediately or even the next season but if the plant/roots get proper care it will eventually come back to life. Planting the vine in the ground rather than leaving it in a constricting container may hasten the recovery.

As an aside, much of my collection of clematis - over 60 - in a former garden were returns to the nursery where I worked for suspected clematis wilt. I took these returns home, babied them and replanted them and virtually all came back to life and productivity.