Campsis

This looks like an excellant place for advice. I have a four year old Campsis Mme Galen. Last August the leaves all turned brown and fell off - suddenly. It has always been a very vigorous plant with lots of blooms. Any idea what happened to it? I am in zone 7, Maryland, near DC. Nothing else around the vine was sick or died. The spot is a low spot and the drainage is not the best in my garden but the plant went from very robust to no leaves. When this spring came, a small piece of it finally sprouted, so it is not lost. As soon as I saw the new growth I fed it heavily. I don't think it will bloom at all this year. It has lost 4/5 of its former growth. I have a terrible suscpicion that the groundkeeping crew in the neighboring area may have hit it with some kind of herbicide where it hung over my fence, but I can not verify that. Is there any 'scientific' way to determine what happened to it or to assure that it regains its former glory?

thx
Joe