Dying tree in a pot

Sarah H

6 years ago I was given a cutting of a curly willow from my grandparents garden.  Initially I planted this out and it didn't do well at all so I moved it to a pot.  The last few years it had been thriving, and about 18 months ago I moved it to a larger pot.  Again, it's been doing really well.  This year however it's not come into leaf and the majority seems to have died.  It had some green sprouting leaf at the base of the tree, but now this is going brown and dying.  I trimmed some of the tree back last week - I don't think this has helped.  I have been feeding and watering the plant.  I am really keen to rescue it if at all possible.  It's currently about 4ft, the plant pot is about 75cm by 75cm.  Any advice out there???