The trouble began about 8:30 p.m. Thursday in an electrical vault that powers the airport's runway lights. The lights flickered out and officials shut down the South Side airport for safety reasons, the Chicago Tribune reported.
About 85 flights were affected with incoming planes diverted to other airports, a spokeswoman for the Chicago Department of Aviation said.
Departing aircraft were eventually sent back to the terminals where passengers deplaned.
Electrical power was restored to the airport's runway lights shortly after 1 a.m. causing delays for scheduled Friday morning flights.
Passengers were advised to call their airlines to check on the status of their flights.
A power company spokeswoman said utility crews determined the outage was caused by an equipment failure at the airport, the newspaper said.