I'm afraid I don't know, but I see the same with several hydrangeas that I planted last summer. A couple have some new growth down at ground level, but the existing branches/stems look dead. They had green growth there during the early February warm up (which was extreme and lasted a long time in this area - a week and a half probably) but now they just look dead.

I also have a hedge of hydrangeas which looked dead for most of the spring. Only this week am I seeing some growth. I guess mother nature is confusing us all.

I'm seeing growth on all of mine now too, finally! Some are coming back at the bottom, while others are finally greening up on twigs that looked dead up until the last week or so.

Glad to know your hydrangeas are up. I am still waiting for my Budds. & Mums. We had very heavy rains today, so maybe tomorrow I'll see something. Unfortunately, my hydrangeas are all infected with some kind of leaf roller. Now I have to take care of that. There's always something!