Is it one that dies back to the ground every year (doesn't sound like it, as it is 15' high) or does it sprout from old growth? If it is one that grows from old growth, it might still have a chance if it wasn't cut down to the ground. I would think that most clematis would also sprout from the ground level too.
Those are my thoughts. I suspect it will survive but might take some time to get back to it's 15' again. Hope it works out and stay optimistic!!!