Most clematis have a year round presence in that the vines/stems at least are always present. If the entire plant has disappeared then chances are something associated with winter has happened to it. If the vines/stems ARE present but no buds developing yet, this again could be related to winter cold.

However I would not be quick to necessarily apply a death sentence :-) Clematis have an uncanny ability of resurrecting themselves from the almost dead, sometimes even a year or two after all visible signs of life have vanished. My suggestion is cut it back if necessary and treat as normal and be patient.

Occasionally I will have a plant that completely disappears, not even an old stem showing. Sometimes it will come back the same year. Sometimes I see nothing for a year. So, agree, be patient and wait before declaring the plant dead. I have seen someone replace a supposedly dead plant with a new variety and the next year have two clematis in the same area, the old one and the new one.

More than once I've given up a clematis for dead, only to have it return in the spring. I'm not familiar with any other type of plant that does this, at least not as much.