Help please, lemon tree..

Could any one please advise why our lemon tree is suddenly looking like this? The only odd thing was none of the fruit ripened over summer, all the lemons were just green, so we did not harvest any. I just harvested about 30 lemons off it a lot smaller than it would normally grow. The tree is about 10 years old, it does have weed mat around it from day one, so I would not it that would be the issue.