Thankfully it happens in daylight whereby the protagonists have time to react and prepare themselves for night. The nuke detonated in the atmosphere, causing a massive EMP that knocked out all of Japan's electricity. All of the other nukes got shot down, but the ship that was supposed to take out the last one was already zombified itself.

Happens in Highschool of the Dead when a submarine fired an nuclear missile as a collaborated attempt to deal with the zombie problems.They wanted to watch us turning them back on again.' ' They didn't want to turn out the lights. Assuming that safety features such as fuses and circuit breakers failed to work (or were bypassed), the excessive electrical current would still make the building's wiring catch fire long before the utility would see any measurable drop in voltage. Household electrical systems (and even small commercial systems) simply do not have the ability to drain enough power from the utilities to cause a system-wide deficit. In Real Life, such incidents are usually caused by natural disasters or large-scale accidents. If it's a really big one, there also tends to be a satellite view of a continent. Often illustrated by a distance shot of an entire city at night (blackouts never start during the day on TV), with large rectangular sections of the city going dark one-by-one, about a second apart.

Alternately, it could be caused by an Electromagnetic Pulse, a Solar Flare Disaster, or perhaps an Alien Invasion. Sometimes the power is adequate and the machine works, other times it begins a quest for the heroes to find something that is powerful enough. This is usually caused by a machine that sucks way too much energy. A sudden blackout cuts the electricity to an entire city, state, country, etc.

Alien blackout tropes