Dig a hole next to the pad's two sunken corners with a shovel. Make the holes slightly wider than the shovel blade and 6 inches deeper than the pad's depth. Save the soil you dig out in a wheelbarrow. Set a brick next to each hole, on the side furthest from the pad.
Work a 5-foot-long pry bar underneath the pad in each corner and rest the pry bar's handle on the bricks. Ask a friend to push down on the pry bar at one corner to lift the sunken end while you push down on the pry bar at the other corner.
Insert bricks underneath the pad and adjust them to hold the pad at the appropriate height. Shovel pea gravel underneath the pad, around the bricks. Walk across the concrete pad to help settle it and add or remove pea gravel as necessary.
Fill the holes you dug with the soil you saved in Step 1. Tamp the soil down with a hand tamper and add more soil as necessary.