What I was doing: I was looking at my war activity since I just done a ground battle assault.
What Happened: when I finished looking at the war activity, suddenly I get a notification saying my cities aren't powered so I go to check If there is enough power plants in each city, and when I check this I see that I have enough power plants in each city and when I check my revenue I'm also making enough oil to power the power plants.
Link: https://politicsandwar.com/nation/id=141904