Is my phormium dead or diseased and have I don't the right thing?

I moved into my house about 4 years ago and in the garden are these very large phormiums, two are green with yellowing leaves which I have cut back on one totally. The other is huge and wide and the leaves had fallen on ground, bent, gone in different directions and dies so i cut back but the bottom is black and water logged so just pulled off the leaves and its formed a black hole in the middle exposing the roots which all look black. The soil is very damp and so are the roots. Have I hacked off to much or is it dead or diseased and what shall I do? Im so worried as it was so beautiful last year with about half a dozen very long black flowering stalks. Please help?