The power ministry said the outage had occurred overnight. It gave no estimate of when the grid, which serves around 117 million people, would be back in operation.
The ministry said in a statement that a "detailed investigation into the immediate and remote causes of the recurring grid failure is ongoing", as was the process of restoring supply.
While power outages in Nigeria, Africa's biggest economy, are common, a total system collapse is not.
Nigeria has installed capacity of 12,500 megawatts but on a good day produces only a quarter of that, leaving many Nigerians and businesses reliant on diesel-powered generators. Diesel prices have more than doubled since the start of the year.
The nation's sclerotic power grid, and its precarious energy supply, are often cited by businesses as a key issue hindering growth in Africa's most populous country.
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