A single lane of traffic has reopened as crews work to fully repair a damaged bridge on the Trans-Canada Highway, which the Ontario premier described as a "lifeline" to the area.

The Nipigon River Bridge is the only major thoroughfare connecting northwestern Ontario to the rest of the province.

"It is the lifeline in terms of product and transportation in northwestern Ontario and as people have said, it connects the east to the west and there is only that one route," Ontario Premier Kathleen Wynne told reporters in Toronto on Monday.

On Sunday evening, the bridge was closed after part of the steel decking lifted more than 60 centimetres above the rest of the structure.

"Apparently, the bolt that holds the girders in place, which keeps the bridge level, broke in some fashion and that lifted the bridge up," said Ontario Northern Development and Mines Minister Michael Gravelle, who visited the site Monday.

By Monday morning, crews were able to open one lane for cars and regular weight transport trucks, according to a statement from the Ministry of Transportation.

The temporary solution involved relevelling the cable-stay bridge using 110 highway barriers, which each weigh nearly 3,000 kilograms.

"One of the great things about this bridge is that it is really flexible, and (there are) a lot of redundancies built into this bridge," said Gary Weiss, a contracts engineer for Ontario's Ministry of Transportation.

However, is not yet known when oversized trucks will be able to use the bridge, nor is it known when the rest of the bridge will reopen.

The closure will have a major impact on businesses on both sides of the bridge, Ontario Trucking Association President David Bradley told CTV Toronto on Monday.

"About 1,300 trucks a day would travel across that bridge, and they would be carrying about $100 million worth of goods a day, so anything that impedes that certainly has an impact on both the eastern and western portions of the country," Bradley said.

"The majority of products being shipped over the bridge are food and grocery products," he added.

Bradley said truckers used to go through the U.S. to make deliveries, but the process became a lot more difficult after the events of 9-11.

"The only route now, for most truckers, is across the top of (Lake Superior)," he said.

With traffic limited to one lane on the bridge, Ontario's trucking association is calling on the federal government to push the U.S. to ease cross-border shipping rules, which would offset some of the difficulties associated with the 12-hour detour south of the border around Lake Superior.

The Nipigon River Bridge, which reopened in November after renovations that added two new lanes, is the most expensive crossing ever built in Ontario.

The cause of the damage to its decking is not yet known, but the premier said staff will "get to the bottom" of the structural issue.

"There will be many questions that will be asked about why this has happened," Wynne said.

"We'll figure out what it was and we'll make sure that it's rectified."

Doug Perovic, an engineering professor at the University of Toronto, says the damage may be weather-related.

"It may have ultimately been induced by a large wind gust – a large uplifting force – against the bottom of the bridge," he said.

NDP MP Charlie Angus said in a statement that the bridge failure is a "perfect symbol" of the infrastructure crisis in the North.

"How does the prime minister explain to other G7 countries that all passenger and truck freight traffic in Canada has to travel on one single road through Northern Ontario?" said Angus, who represents the northern Ontario riding of Timmins – James Bay.

"If the federal government is looking to make investments we need to make sure that they aren't just expanding suburban roads and transit. Northern roads must be part of the equation."

With files from CTV News' Peter Akman