Having a few wisteria myself, I would also be concerned by this. I have no suggestions to offer other than based upon personal experience, they are virtually impossible to kill. I'm interested in what others suggest for you.

I am presuming it is in the ground? The first thing that I thought of was lack of water.....maybe it hasn't rained enough to give it a good soak.They are quite tough, hope it starts to grow in the spring.

Have you checked for Wisteria scale, this over the last few years has been increasing, i carnt see it being due to lack of water, they are normally very resilliant to dry conditions.

Check for scale generally together with wisteria scale, as Julien suggests. I also suggest you inspect the graft union for signs of damage, rot, or any obvious problem. This has died back so suddenly the cause is more likely to be one of three things - failure of the graft union; honey fungus problems or phytopthera at the roots, caused by the soil being too wet or waterlogged.