It may have died from too little water, but I'd keep watering anyway.

I have a couple that I cut down every year in very early Spring so they grow as
bushes (and third one trained as a small tree).

Last year I cut perhaps a bit early and one of them seemed to die - Spring came
and nothing on that one. A month or two later, it miraculously came back and is
now almost as high as the other one. Could have sworn it was dead - upper wood
shot, bark slimy...but something was still going on underground.

After all, they really do have the vitality to warrant the description of
'large weed'.

Good luck.