You search the area, put up posters, check local shelters for your pet, etc. Beyond that, is the pet sitter a professional that carries liability insurance? If they are, you should be able to lodge a claim due to negligence and receive money for damages (your pet, your time, etc). If not, good luck. Even if you sue and win, the likelihood of actually collecting a judgment from an uninsured, low paid, self-employed, pet sitter is pretty near to zero.