A dent in a stretched canvas is something that happens to most painters at one time or another; a finished painting or an unpainted canvas can be dented in storage or shipping. These dents look alarming but can be easily removed and the canvas will be completely back to normal.
Wet the back of the dent
Removing a canvas dent is a quick and simple process of wetting the back of the canvas just where the dent is. If the canvas is dented from the front to the back or the back to the front, the process is the same, you wet the canvas on the back. It works for unprimed, primed, and painted canvas.
You can use tap water in a spray bottle or hot water from the tap. I have heard of people using steam from an iron or a kettle. They all work instantly to reshape the spot. My usual method is hot water from the tap, dribbled on the spot. I have never had a negative side effect from water, room temperature, or hot. (See below for cautions if you have sized your canvas yourself with rabbit skin glue.)
The water will probably bead up a bit so you will need to give it a quick rub with your fingers to encourage it to soak into the weave. The dent should pop back right away. If you get too much water on the back it can run into the stretcher bars. Wet stretcher bars can warp as they dry, so have a towel on hand to mop up any excess. You just need the water right on the back of the dent, not all over the canvas.
Spray or dribble water just on the back of the dented area. Rub with your finger if needed. This dent disappeared the instant the water touched it.
After the excess water was wiped away, you can’t tell there was ever a dent.
Beware of using water on rabbit skin glue sized canvas dents
If you have sized your canvas with rabbit skin glue, water on the back might soften the glue as it will absorb the water. So don’t use hot water and use as little water as you can, being quite careful. But acrylic sized or primed canvases should all be fine and bounce back nicely.