The shutdown of Line 5 would remove something in the order of 60% of the refined product supply for Ontario and, therefore, significantly increase prices. Hopefully there are ways in which some products, like gasoline and aviation fuels, etc., can be brought into the province by rail or by truck, but that will definitely be more expensive.
The ultimate answer to a permanent shutdown of Line 5 is the construction of an additional pipeline into the area. That would clearly take years to achieve, given the difficulties that other pipelines in Canada are having with permitting processes today, so it would probably increase prices considerably.
One of the things that's always been typical of transportation fuels is how energy inelastic they are. They are relatively unresponsive to significant increases in prices.