Nigeria's national electricity grid collapsed on Sunday, the Transmission Company of Nigeria (TCN) said in a statement.
Power outages in Nigeria, the most-populous nation in Africa, are common, but a system collapse is unusual.
"At 11:25am today, the nation's electricity grid experienced multiple trippings, which led to the collapse of the system," the company said in a statement.
"TCN has since commenced grid restoration; power has been successfully restored to every part of the country, except Calabar, Ugwuaji, Makurdi, Jos, Gombe, Yola and Maidugiri axes," it added.
TCN said it would conduct investigations to establish what caused the "multiple trippings" as soon as the grid was fully restored.
The nation's sclerotic power grid, along with the resulting precarious energy supply, is a key issue hindering growth in the continent's largest economy.
Nigeria recently implemented its first power tariff increase in state-controlled prices since 2015. That doubled prices for some consumers, but the government and industry said it was needed to allow distribution companies to recoup costs and pay generating companies.