From the official press release of How to Succeed:
"The name of the deceased and any further details are being withheld by the authorities pending notification of the family. The incident was not caused by an accident related to the production. The cast, crew and theatre staff mourn the loss of this member of their theatrical family."
My thoughts/condolences go out to the cast/crew family of the crew member and everyone working at the Al Hirschfeld Theatre.