The first thing to do is call your local animal shelter. Give them a description of your dog. Call every day! Better yet, visit the animal shelter every day. Often shelter workers are busy and miscommunicate information.
Be patient, it can take weeks to reunite a canine with his family, but remain diligent in finding your pet.
Don't replace your pet right away. I once identified a large lab from a local animal shelter and contacted his owner. His family had failed to check the shelter. When I contacted the family, I was told that they "didn't know what had happened to him" and had "replaced" him with a puppy. His family had not even bothered to check the local animal shelter!
Shelters hold dogs for three days. If your pet is missing, you should take immediate action to find your pet.
* Owner relinquished pets do not have to be held for three days. Owner relinquished dogs do not even have to be offered for adoption. Owner relinquished dogs can be euthanized the minute you sign them over. I have seen owners bring in their pet with his cute bedding with his toys and reliniquish him to a shelter - a shelter is not a home away from home and most shelters are not 100% No Kill. If you must relinquish your pet, please be responsible and find him or her a new home. At least locate a 100% no kill shelter or a 100% no kill rescue group. No kill facilities are usually packed to capacity and there may be a waiting list. Allow time to rehome your pet.
Take a picture of your pooch NOW and load it onto your computer so you will have it in an emergency. If your pet is lost, make flyers and distribute them throughout your neighborhood, to local animal shelters and to local rescue groups. Run an ad in the newspaper. Place "pet lost" signs throughout the area. Pets can cover a lot of ground when they get out. They often become confused when trying to find their way home. Do not assume that your pet will find his way home on his own.