Q.Over-pruned Hydrangeas. Now What?
Two years ago, during winter, I over-pruned my Hydrangeas, from 6′ down to 2′, expecting new growth the next spring. Leaves appeared, but no new stems. I waited, and this year it’s the same thing– healthy-looking leaves on the old wood, but no new wood stems. Are these hopeless or is there a way to revive them?

Certified GKH Gardening Expert
Unfortunately, this depends on the type of Hydrangea. It does look like it is trying, but it might be too damaged to go back to what it was. Prune away all growth that isn't alive or isn't making new growth. This will help clear the way for any shoots that might take over. Make sure to feed them with an all purpose fertilizer. I'd do half strength until you see signs of recovery.
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea

