Can anyone tell me what maybe happened on this flight?

Respectfully, I know nothing about planes or aviation. This was on a nonstop international passenger flight from CHI O’Hare to HND Tokyo. The flight was about three hours in and turned around for an emergency landing. When they landed there was a large emergency response standing by. This plane landed at an airport then all passengers were offloaded, then sent back to Chicago to rebook a flight for today, a day later. This has been a nightmare travel situation.