If caught early, most concrete slabs, in descent condition, can be lifted back toward original position quickly and cost effectively.
You may find yourself without the repair option. If your sunken concrete has become severely cracked you may not be able to repair it, since repairing will most likely involve raising it. Raising a severely cracked or broken concrete slab may not be possible. Neither would it be practical to raise a slab with portions of the slab that may separate when raised. To decide whether or not to raise your concrete slab you'll need to determine if it is in a single, solid piece that will likely not fracture when raised.
Another point to remember is, WHY did the settlement occur in the first place? When replacing concrete, it is important to make sure the soil base is solid. Otherwise, your newly poured concrete may move and settle again in the near future.