Frying pans are made from a variety of materials and are subject to warping due to the temperature extremes used during cooking. A warped pan doesn't sit snugly on the stove burner; it wobbles around and conducts heat poorly. You can repair it by tapping the bulge back into place. This can be a short- or long-term fix, depending upon the type of pan. For an expensive pan, it may make more sense to take advantage of the warranty and return it to the manufacturer rather than trying to repair it yourself.
Lay a towel or rag on a firm, flat surface such as a work bench, a kitchen counter or the floor.
Place the warped skillet on the cloth open side up if the warp is convex, or facing up, or open side down if the warp is concave, or facing down.
Place a second towel or rag over the bulge to protect the pan's surface.
Tap lightly on the warp at first with the hammer or mallet until it returns to its original shape and increase the force if the bulge is stubborn.
Inspect the skillet on the other side and repeat the procedure until no bulge appears on either the inside or outside of the pan.
Remove the towels or rags and place the pan on a flat surface to test for evenness. Repeat the procedure if the pan still wobbles.
Wipe out the inside of the pan if it is coated with a nonstick finish to remove any bits of coating that may have come off during the repair process.