wisteria woe

my once healthy wisteria started to go pear shaped last year, all the leaves went yellow and fell off, and a lot of the vines died off, I dont know what the problem is so I dont know how to solve it. The plant is mature we bought it 6 years ago already a large plant in a tub and planted it in the garden, it was fine until last summer, I dont know whether its the soil (heavy clay) lack of water, feeding ,pruning, disease, or heat (40 degrees) now only a few shoots coming from the ground at the base seem healthy.