Q.Hydrangea – No Flowers, Leaves Withering And Dying
I’ve checked the base of the main stem and there seems to be small bore holes made by an insect. I’ve read that you should cut off anything above the bore holes but if its at the base of the main stem can I cut back most of the plant and it will still grow back?

Certified GKH Gardening Expert
Usually borers are in the branches and you can cut off the affected branches. I don't find any source that says hydrangeas are bothered by borers, so be sure that is what you are seeing.
Cutting back your hydrangea to the ground will revitalize it and it should come back. However, that is best done in the spring before new growth. But if you have borers, that is your only non-chemical option.
https://ucanr.edu/datastoreFiles/268-86.pdf
https://agrilifeextension.tamu.edu/library/landscaping/wood-boring-insects-of-trees-and-shrubs/