The power outage was reported around 5:30 p.m.
Southern California Edison initially reported it was caused by metallic balloons coming in contact with wires. The utility later corrected that statement, saying there was a different outage in Long Beach that was caused by balloons. The agency is still investigating the cause of the outage at the port.
Edison said 429 customers were affected by the outage.
One truck driver said he and his wife, also a truck driver, had been stuck at the port since the afternoon, with little information coming to them from port officials.
"We're trapped," the driver told Eyewitness News. "Nobody in the terminal cares that we're trapped. Very frustrating day. A wasted day."
The agency expected to restore power late Thursday night.
The Port of Long Beach is the nation's second-busiest container port, right behind the Port of Los Angeles. It handles some 82 million metric tons of cargo every year.