What zone are you in Nannygoat? I had to google tree hydrangea - is yours Hydrangea paniculata?

Here in the UK hydrangeas are regarded as hardy, but I had a guest from Germany some years ago who was amazed to see ours growing out in the garden, as where she lived they are grown as tub plants that have to be brought under cover in winter.

Some species of Hydrangea are considerably hardier than others, so it really depends on what this particular 'Tree Hydrangea' is.

Even when cut back to ground level they are capable of making quite rapid regrowth, providing the roots have survived, so patience is the key!