I am a former CM and I remember when something like this happened, although luckily it was just limited to Hollywood Studios where I was working. I was in the employee cafeteria on the phone with my supervisor (I was finishing lunch and they were about to give me my new assignment) when the power flickered off for maybe two or three seconds. I thought it was just the cafeteria but my supervisor across the park said it happened over there too. I left the backstage area and a little while later ran into a guest who asked if I knew if Tower of Terror was back up. I told them I wasn't aware it was down and they told me that apparently every building had an outage and it was enough to shut down every ride in the park. Later I remember standing in front of the wait times board as another CM was one multiple radios and updating the board as each individual attraction went back up. He was like an 80s Wall Street stockbroker as he shouted out information. Tough day but luckily everything was able to get back up and running fairly quickly, and surprisingly the guests I talked to were pretty cool about it. Stuff just happens, unfortunately, even at Disney.
And in a similar story, I also worked at Universal Studios. There was a big construction project happening and someone did something to knock out the power right as the park was opening in the morning for the resort guest's extra hour. They had to open the gates and then tell everyone that none of the rides were working! I worked at the front of the park and luckily a bunch of us scrapped together some stuff like hula hoops and bubbles and kind of turned that main street into a street party. It at least kept the younger kids distracted for a bit! People were a little more annoyed at that instance since they did pay extra for this perk that they now couldn't use buy luckily the attractions did eventually get up and running pretty quickly.