The accident at the Santa Clara construction site occurred around 8 a.m. Monday. Additional details were not known.
Calls to Santa Clara fire officials, the state workplace safety division and the 49ers were not immediately returned.
The stadium is scheduled to be finished by July 2014. An elevator mechanic, 63-year-old Donald White, was killed at the construction site in June when he was struck by a counterweight while working in a shaft.