Why don't you just watch the entire film instead of being lazy and asking questions on here?

As an electrical engineer that was a totally stupid premise for the movie. Power goes out and stays out for years? No way. Just won't happen. One thing that could cause a complete world wide loss of power is a geomagnetically induced current. It might damage some equipment somewhere, but they plan for that and could restore power fairly promptly.
https://en.wikipedia.org/wiki/Geomagnetically_induced_current

it's never explained. but it doesn't much add up because they can run power through their generator. honestly, the power grid is just a giant generator system run on oil, so there's no reason power couldn't be restored globally in short order