Q.I Have Over-pruned My Apple Tree, There Is Now No Canopy Left, How Do I Save It?!
The tree is mature, been here for about 10/15 years and produced loads of apples last year.

Certified GKH Gardening Expert
I'm afraid you will get a bunch of waterspouts. If all the leaves are gone there is no way for the tree to produce food for the rest of the tree.
The only thing you can do is make sure you take care of it properly with correct fertilization and watering to help it heal. Here is more:
https://www.gardeningknowhow.com/ornamental/trees/tgen/damage-from-over-pruning.htm
https://www.gardeningknowhow.com/garden-how-to/info/how-to-correct-pruning-mistakes.htm
https://www.gardeningknowhow.com/edible/fruits/apples/pruning-apple-trees.htm
https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-feeding.htm