It has had adequate water and fertilizer, yet the leaves turned brown, beginning from the top. It does have new buds on it. It had been shaded by the wisteria on top of the pergola but we had that removed last fall so it now has sun most of the afternoon. Temps were in the high 90"s since May here in N. C.
It sounds like you may have a fungus. There are several that it could be causing the issues, but treating the hydrangea with a fungicide will take care of most of them.
It likely will not recover its leaves this year, but it should be ok for next year. Make sure to clean up any leaves that fall from the plant and destroy them so that they do not harbor the fungus over the winter.