If you've been watering and it didn't help the leaf droop then something could be wrong with the roots - either root rot or something perhaps girdled the main trunk and/or dug under and/or chewed the roots . You might want to check down near the ground to see if anything was chewed... maybe consider getting some cuttings just in case. Alternately, if anything was sprayed in the area (herbicide, pesticide, etc), it could be getting impacted by that.

A picture might help to see better what is happening and you could also send a branch to your County Extension office to better diagnose the problem.

Yes to what Jenny said...do some detective work. Here's a fact sheet that will help as well. Some extension services still make house visits, so that might be worth a call. Taking a cutting someplace is probably not going to help. A diagnostician needs to see the whole plant and planting site. The symptoms you describe could be caused by many different factors.

Here is a link that might be useful: Camellia problem fact sheet

Besides pest problem at root area or possible tiny spider mite under leaves, there are two common situations:

The plant must be in the ground. Is the area very wet most time? Wet root and over water can promote root rot. I have seen in wet field of camellia plantation in Sichuan, China. They dig trenches at either side for drainage.

Too much sun and poor soil can also cause problem. Even you water the plant but the root gets little water. Prolonged condition would cause the root branches of deterioation and poor plant.

With all the common sense, I think you can fix the problem.