An established old camellia is nearly indestructible. They can take a bad haircut and always come back. (I have even cut them back to a stump in some instances to rejuvenate.) I would suggest patience, to see how and where it generates new growth, and then prune for shape. It may take a couple of years to regain its form, but it should be fine.

Don't worry. It will come back and be even fuller. A bush that old, unless it's a slow grower like Pink Perfection, will be back to it's full height in about 5-6 years.

I "inherited" a bunch of camellias that are about 20-25 years old. The healthiest looking ones are the ones I've pruned to stumps. It really only takes 2-3 years to get a bushy plant back.