Too dry perhaps? They do drop leaves even though evergreen, as they make new ones. My 25 plus year old Camellia has lost some leaves today......cos I got the pruning saw out! My brother cut one right back in his front garden that wanted to share the living room. It sulked a bit then put on a stunning display of large blooms. I suppose you might know that this is the time to sort em out and feed them, to promote budding up, and get them the shape YOU want.

All the leaves gone does sound like drastic drought, though, or maybe serious root damage. Is it in a pot, or in the ground?

I hesitated to suggest it might be dead!!!!! I do know they can play dead, as I had one that did that. When it was young. It's still alive(more than 10), but certainly not vigorous, but it puts on a good show of huge flowers. My brother scalped his camellia, now it looks marvellous.

Check the twigs. If they are still flexible, and not brittle dry, and they show a moist, green layer just under the bark, there is still hope.