As sad as this sounds, many hydrangea only have finite lifetimes. Various conditions apply - yes, rodents could've bored through the root system, etc. But generally speaking, macs need to be pruned back heavily (they say the outermost 1/3 of the shrub) every 5-10 years. Dividing off some of the crown is also feasible - you can sell or give away those parts. Also, dead canes should be thinned out yearly.

Yes, it's looking like you'll have to hack away the old crown and start fresh with the baby.

Sorry to hear the bad news, Carol. You may also want to do a soil test if you have not done one recently and check salt levels. Do you think it may not have gotten any moisture this winter? Our weather was pretty dry last summer but Fall & Winter were equally unusual with rainfalls once a week on almost all weeks thru March. That got me wondering if your weather might have been the opposite, very dry. I almost killed a camellia several years ago when I turned off the sprinkler system due to frigid temperatures and forgot to turn it on again.... sigh... hee hee hee! Nigra is a nice hydrangea too by the way!