Toronto Hydro Chief Executive Officer Anthony Haines vowed crews won't stop working "until all the lights are on," the Canadian Broadcasting Corp. reported.
The layer of ice that coated the Canadian province initially knocked out power for more than 350,000 customers, prompting Toronto Mayor Rob Ford to call it "one of the worst storms" in the city's history.
Up to an inch of ice accumulated on road surfaces and trees in the greater Toronto area, causing power outages for about 250,000 customers, the CBC said. Another 100,000 customers were in the dark in a swath from Niagara Falls to Kingston, Hydro One said.
The power company said some customers could be without electricity through Christmas.
Temperatures were to drop into the single digits Monday night and hit a high of just 14 Tuesday before rebounding into the 20s and 30s the rest of the week, forecasters said.
Though the storm had passed in Ontario, officials warned roads could be slippery due to lower temperatures forecast by Environment Canada.
"There could be quite an extended period of freezing rain right through Monday" in the Maritimes provinces, said Rob Kuh, a forecaster with Environment Canada.
Flights at airports in Toronto and St. John's, Newfoundland, were delayed or canceled, and routes on Via Rail were expecting delays, the CBC reported.
Two Toronto hospitals had power restored Monday after relying on backup generators during the storm, the CBC said.
Subway service was restored in Toronto Monday as well, except for the Sheppard line.
The specter of facing days without electricity sent many people to the homes of friends, relatives and neighbors, while others sought shelter at police stations or booked rooms at hotels.
The Royal York Hotel, usually going through a quiet holiday period, had all 1,365 rooms filled, the CBC said.
York University and Humber College called off classes, and the Toronto Zoo, normally open every day except Christmas, was closed because of the inclement weather.