We were just about to board Men in Black about 945am when all the lights went out. Emergency lights came on in about 10 seconds, and everyone waiting in line was escorted out. I suppose those on the ride needed to be evacuated. The lockers outside MiB weren't working either. I'm not sure how long it took for people to retrieve their belongings. We waited around 15 minutes and then gave up, since we only had a Coke bottle in the locker.
Gringotts was working, so was The Simpsons. A friend planned to join us, but texted us to say that everyone was being directed to IoA instead. But by the time we walked to the exit, they started letting people into the Studios side again.
As far as I can tell, Despicable Me, The Mummy, and Transformers never reopened by the time we left about 2pm. Lots of shops and restaurants were dark too. At an outside drink kiosk, I saw an employee writing down purchases on a waitress pad.
We had lunch at City Walk and then went to IoA where everything was operating.
Oh, yeah, there were several helicopters flying around overhead. I guess that the power outage was "news." I saw a brief report on one of the local stations last night.