Levels of power being generated fell to zero megawatts early on Thursday.
Some supplies have been restored but only a tiny fraction of normal consumption in Africa’s most populous country.
Power supplies are often erratic in Nigeria, despite its role as a major oil and gas producer.
The West African country’s grid collapsed at least four times in 2022 – authorities blamed this on technical problems.
The theoretical maximum amount of energy Nigeria could produce is 12,500MW, but the country normally produces a quarter of that, the Reuters news agency reports.