japanica tree

[david]

I have a lovely, very old japanica tree that all of a sudden lost it's
leaves.  It's been two years now and the tree is still alive, but it only
occasionally sprouts a leaf or two.  It's in a well drained location,
plenty of ventalation, and receives sun about half of the day. The height
is about 20 feet.  I tried prunning it back but to no avail.  It's been
doing fine for years, but the last two years I have noticed this problem. 
I live in Charleston, SC.  Does anyone have any ideas or suggestions? 
Thanks so much, David