I have a 5gallon Australian Red finger lime that had similar issue 8wks ago. It was doing well, very green, lots of flowers so I moved it from under my shade tree to give it more direct sun. In about a weeks time the tree completely dried up (wrinkled green leaves) so I thought it was dead. But I just cut off all the smaller branches with leaves still attached and left some skeleton branches. I moved it back into a shaded area and watered it as if it was normal. After 4wks it started to show a few new buds, now all of the small branches that were cut back have new leaves and it looks like it will survive. Funny thing is, one week after my finger lime took a dive, my sumo/shiranui that is in a large 40gallon pot did the exact same thing. Its leaves looked like your tree, all brown but did not fall off. So I did the same thing but I had to cut all the dead branches and there was only 18" of the main trunk left. Now after 6 wks the sumo is actually coming back, saw some green leaves growing out again. Very strange since this sumo has never had a problem and I had it in the same place for about 4 yrs.