Hi there. Really hoping someone can help me!

We moved into our house last summer, on our property we have a huge hydrangea that's about 6 feet tall . I'm not sure what type it is. The blooms start out a light green and turn bright white. I will take a picture of the blooms tonight and upload it if that will help.

Anyways, it was a mess last year with big trees growing in it from the previous owners. It looks like she wasn't able to remove them, so she just trimmed the branches of actual trees growing in the middle of the hydrangea. This also occurred with all the lilac bushes on our property.

So this spring my husband and I got a hand saw and cut down the trees as low to the ground as we could in all the bushes. Yesterday I went out to trim the branches on the hydrangea to clean it up a bit and realized that the very base of all the large branches are rotted. You can literally just take your bare hands and take chunks of the trunk off. The strange thing is, it looks very healthy higher up. The tops of the branches are healthy, and it's in full bloom. It's just covered in blooms.

Is there anything I can do to help it? Should I cut it down to the ground and let it grow back? Any advice would be great. Here is an upclose picture of the base and I'll take a picture of the whole hydrangea tonight.