This Bigleaf Hydrangea (H. macrophylla) has new growth at the base, but the stems are bare – a classic example of dieback.
If you’re seeing bare stems in mid-May, it’s ok to cut them all the way back to the base because they will not produce any flowers this year. The new stems should grow to the same height as the old stems. Hydrangeas that flower on new wood (H. paniculata) will still bloom this season. Hydrangeas that bloom on old wood (H. macrophylla) may not bloom at all, or may have a few blooms but not until late summer.
For more information about Hydrangeas, visit the United States National Arboretum website.