I cannot blow up the pictures but from what I can see you have been invaded by a variety of insects. I see white flies and I think cottony scale. You have sooty mold so there may be other things I cannot see. You have some green limbs still so there is hope. Maybe others can see more than I.

Sorry about your plant. Definitely prune anything dead or diseased. Doesn't look good, but I'm no citrus expert. Might just be best to start fresh.

Look more like Mealy bug to me. Plus, more than likely the tree has other root problems.

Thanks all. I gave it a very hard pruning today and cleaned off the trunk, which had a sticky/cottony scale or bug of some kind surrounding it. I’ll fertilize it lightly for now and keep a close eye on it—fingers crossed it comes back.

thanks again!

Pests are hard to irradiate. Just because you don’t see any now, doesn't mean they’re gone. You should regularly spray with HO and wipe done with rubbing alcohol to keep them at bay. I would also clean your other trees and treat as well, even if you don’t see any bugs. They’ve likely spread.

Soft scale infestation on such a small tree usually signifies inadequate levels of sun exposure.. Soft scale loves shade. I can't help but wonder what the tree is doing inside at this season in zone 9b.