The concrete pads under air conditioner and heat exchanger units tend to settle over the years. As long as there's no strain on the copper coolant tubes and electrical lines, and the unit stays dry and level, it's OK that the pad has sunk. But if it continues to sink and pulls the lines tight, or if water puddles around the unit, you'll need to take action.
Try working a long board under one end of the pad, lifting it up, then packing sand under it. Do the same on the other end, making the pad as level as possible. Lift the pad slowly to avoid cracking it. You won't need to unhook or remove the air conditioner. However, if the pad breaks apart, have an AC technician disconnect the unit. Then, break up and remove the pad, and install a new leveled-in one. Consider replacing it with a plastic pad. They're available where air conditioners are sold and online.