An estimated 380,000 homes in Nagano Prefecture lost power early Monday, Chubu Electric Power Co. said.
The three-hour outage also paralyzed a wide range of services, from railways and traffic lights to hospitals.
The blackout began around 5:35 a.m. and power was fully restored by just after 10 a.m., the utility said.
The outage affected wide swaths of the prefecture, including the capital.
Chubu Electric said it traced the cause of the blackout to faulty transmission lines between substations in the cities of Shiojiri and Ueda, and was investigating further.
After some 2,415 traffic lights stopped working at 5:55 a.m., the Nagano Prefectural Police deployed officers to power the lights with generators or to direct traffic with hand signals.
A man in his 70s was sent to a hospital after a collision between a light truck and a car at a crossing in the city of Nagano where the traffic lights were out. Three other road accidents were reported elsewhere in the prefecture.
Services halted on a section of the Nagano Shinkansen Line for around 3½ hours, East Japan Railway Co. said.
JR East said bullet-train services between Nagano and Karuizawa stations were out of service from the first run at 6:03 a.m. and got under way for the day around 9:40 a.m. A total of 15 scheduled services were canceled, affecting about 6,600 passengers.
Some local trains were also canceled or delayed.
With the blackout paralyzing a range of infrastructure and facilities, the prefectural government dispatched all members of its crisis management task force from around 8 a.m. to respond to reports and ascertain the scale of the outage.
At least 12 public schools in the prefecture canceled classes for the day, and 110 more operated on a reduced schedule.
In the city of Suzaka, Nagano Prefectural Suzaka Hospital deferred its Monday morning outpatient appointments because it was unable to access electronic medical records.
Elsewhere in Suzaka, a convenience store owner said people had rushed in to buy breakfast food and batteries.
“We couldn’t use the cash registers, so we had a hard time adding up the transactions with calculators,” the owner said.
A 24-hour supermarket in Nagano opted to close temporarily.
The Sugadaira Kogen snow resort in Ueda opened around 1½ hours later than usual after its chairlifts stopped working, the resort’s operators said. Guests at nearby inns kept warm with blankets in lieu of heaters.
Chubu Electric released a statement apologizing for the impact of the disruption.