Electrical workers determined the cause and began restoring power to some places by late Thursday afternoon, the Syrian Ministry of Electricity said without detailing what had gone wrong.
It wasn't clear how many people the outage affected as many cities outside of the regime's control don't get service from the government-run power grid. ISIS and other militants groups control large parts of the country, and many cities in these areas use fuel-powered generators for electricity.
Efforts to relaunch power service could take two to 12 hours, a Ministry of Electricity official said in a video posted online late Thursday afternoon.
Shortly before the reports of the outage, the ministry said on its Facebook page Thursday that militants had hit part of a power-generating station with rockets in the western city of Hama. The Syrian government hasn't said whether this attack was linked to the nationwide outage; the ministry said maintenance workers were fixing the damag e.
Syria's power infrastructure has been damaged during the war, accounting in part for frequent outages even in areas that it still serves.
Thursday's outage came in the middle of a two-week truce between government forces and certain militant groups -- a pause in fighting that is meant to allow humanitarian aid to reach people who have been cut off by the war.
U.S. officials: Syria ceasefire 'better than expected'
Rebel negotiation coordinator: Truce 'close to collapse'
The temporary truce also is supposed to ease the way for peace talks scheduled to take place Wednesday in Geneva, Switzerland.
But Riyad Hijab, negotiations coordinator for a main Syrian opposition group called the High Negotiations Committee, said Thursday that the truce was "close to collapse" because of combat attacks against the opposition by the Syrian regime and its allies, including the Russia, which is conducting airstrikes in the country.
'We are not talking about violations of the truce, but talking about the continuation of combat operations and war crimes committed by the regime and its allies (through) today," Hijab said.
Hijab said the regime and its allies violated the truce more than 100 times in five days, killing more than 40 people and injuring 92 others.
"They are continuing ... to target populated areas by aerial and artillery bombardment, barrel bombs, cluster shells, heavy machine guns and rocket launchers," he said.
Senior U.S. officials on Wednesday branded as credible reports that both the Syrian regime and Russia have violated the ceasefire terms, hitting targets other than ISIS or al Qaeda-affiliated al Nusra Front, which are not a part of the multiparty truce deal.
But the officials, speaking on condition of anonymity, also expressed cautious optimism about the truce's prospects, with one describing the progress as "better than expected" Wednesday. Another said, "We're seeing the beginnings of progress" on both the military and humanitarian fronts.
CNN's Nic Robertson and Michelle Kosinski contributed to this report.
