The difference in temperature between the hot air inside the skillet when covered with a lid and the cooler air outside the lid can create a vacuum. When this vacuum seal occurs the lid is drawn tightly towards the skillet and some air is removed. In some cases all air is removed and the lid actually becomes inwardly dented from the suction of the seal.
Hold the pan handle (use a pan holder if hot) firmly in one hand. Use the other hand to carefully lift the lid straight up and off. If the lid does not move, proceed to Step 2.
Place the pan back onto a stove burner. Turn the stove burner to low heat and allow the pan and lid to heat up for a few minutes.
While holding the pan handle with one hand, grasp the lid handle, which may be warm to the touch, and pull upwards to release the lid. If this doesn't work, move on to Step 4.
Remove the pan and lid from the heat and allow it to cool. Place a towel wet with hot water on top of the lid for a few minutes.
Repeat these steps as needed until the vacuum seal is broken. Let the lid cool down between attempts to prevent it from becoming too hot to touch.