Energy Minister Khurram Dastgir told reporters an inquiry had been launched into the outage, which began at around 7 am (0200 GMT) and has so far lasted more than 12 hours. "We have faced some hurdles but we will overcome these hurdles, and will restore the power," he added.
The outage, which the minister had said was due to a voltage surge, is the second major grid failure in three months, and adds to the blackouts that Pakistan's nearly 220 million people suffer on an almost daily basis.
Analysts and officials blame these power problems on the ageing electricity network, which like much of the national infrastructure, desperately needs an upgrade the government says it can ill afford.
The International Monetary Fund has bailed out Pakistan five times in the last two decades. Its latest bailout tranche, however, is stuck due to differences with the government over a program review that should have been completed in November.
"There's an underlying weakness in the system," said an energy ministry official, who declined to be named as they were not authorized to speak to the media. "Generators are too far from the load centers and transmission lines are too long and insufficient."
Pakistan has enough installed power capacity to meet demand, but it lacks resources to run its oil-and-gas powered plants. The sector is so heavily in debt that it cannot afford to invest in infrastructure and power lines. China has invested in its power sector as part of a $60 billion infrastructure scheme that feeds into Beijing's "Belt and Road" initiative.
"We have been adding capacity, but we have been doing so without improving transmission infrastructure," said Fahad Rauf, head of research at Karachi brokerage Ismail Iqbal Industries.
The outage occurred across swathes of Pakistan on a winter's day where temperatures are forecast to fall to around 4 degrees Celsius (39°F) in the capital Islamabad and 8 degrees Celsius (46°F) in financial hub Karachi.
Many people also have no running water because there was no power to the pumps. "People are suffering badly from this power cut," said Sagar Pahuja, a water and sanitation officer at the municipality of Jacobabad, a southern city with daily scheduled power cuts.
Earlier, Dastgir told Reuters that supplies were being partially restored from the north to the south and that the grid should be fully functioning by 10 pm (1700 GMT). It also took hours to restore power after the last major outage.
The outage hit Internet and mobile phone services. Several companies and hospitals said they had switched to back-up generators, but disruptions continued across the board.
"If this power outage continues for 10 or 12 hours, it will cause great losses," said Nassim Shah, a commuter in the northeastern city of Lahore, where the outage halted the metro network. "We hope the government will restore power soon."