Can you post a picture?
If it's a folded crack, it depends. Is the canvas coated or still matte?
Coated you might be able to cover the area with oil pastels, by gently applying the pastel into the crack. Note if you get this to work, you will need to seal the oil pastel, I use clear nail polish. I use the oil based pastels since they will stay in place and fill the crack. If it doesn't work right the first time, it's easy to wipe out the pastel and try again. NOTE if the canvas is a like Breathing Color's Crystalline, you can't do this as the oil pastel will attack the ink nearby and lift it off. If you coat the Crystalline with Glamour II or Timeless, then you will have no problems.
If matte and not coated, you have a lot more leaway, and you should be able to find a fine art marker that will blend in the area around the crack. If it's a light solid this might be harder to do.
With a small spot you can sometimes get a fine art maker to work on glossy or a coated canvas. You have to very careful as you apply the marker as too much pressure and it might create a ring, where the ink hits the coating break. You also have to be very care to test the color on a white piece of the canvas as it's very easy to get the fix too dark.
Paul