I've planted three Limelight hydrangeas last year.
I will add that when I planted these last year, the struggling shrub hadn't gotten enough water down into the plant early on and I didn't notice it until I'd almost lost it so, it has been a bit disadvantaged from the start.
It does sound like the root system was damaged. You might dig it up, remove any dead roots and replant. It is quite late for this however, so ideally it should be done in very early spring. Or you could dig around a little of the plant and do this. Be careful of overfertilizing an ailing plant! ep