Why is my old Willow dying?

If the bark is loose and spongy, can you pull a bit off to see if there's anything inside or underneath, and to examine the wood underneath? This sometimes happens from strimmer damage - don't know if you've used one round the tree last year? If you find anything, re-post what you see please and we'll all have a go at helping.

You don't say what type of willow. It sounds like a pretty old tree. Old enough to start to decline and die maybe? I would prune off any dead material, and encourage any new and living growth. Get what you can out of it in its last days. It's difficult to know what you mean by the bark at the base being loose and spongey. Would be good to have more info, or a pic. Is hard to know what's going on with the tree without determining what's really happening there.