In the course of two days last week, my wisteria went from healthy looking green leaves to shrivelled and falling off. It looks like it has been sprayed with something! It has been very healthy for the last 28 years and is about 15 foot long and 8 foot in depth. Because it is on the north (back) of the house, it is always late in flowerng. Have you any idea what can be wrong for this to happen so suddenly?
It sounds like it may have been hit by overspray from someone spraying weedkiller. Were you or a neighbor spraying anything like that recently? The wind may have carried it to the plant. If the leaves died, there is still a chance that the stems have survived. If only the leaves died, it will grow back. I would spray the plant down well with water.
The other possibility is that you may have a critter in the roots that is feeding on them, so I would check the roots for any signs of damage.