In spite of the best care and training, some dogs run away. Sometimes they're frightened and want to get away from something specific, like loud noise (especially an owner yelling at them). Some dogs just yearn to run as far and as fast as they can. And then, sometimes there's something "out there" that they can't resist - like garbage! Beagles and Bassets are known for bolting on trash collection day. They can smell the goodness in the air! I used to have a Cocker Spaniel who raced me to the gate every Friday morning,'cause he knew what goodies all those curbside containers held! Then we'd do this foot thing with him trying to bolt between my legs and me fending him off with my lower extremities. He occasionally won; the good part was that he never got further than my next door neighbor's goodie bags.
What to do when your dog runs away? If you're there and see her go, grab her favorite food or treat and try to get in front of her and lead her to the treat. For example, you can jump in your car and go in the same direction as your dog, but go ahead of her; stop, open the car door, and try to lure her in with the treats. It's best to offer something that smells good. I've used cheese and raw meat in the past.
If that doesn't work, don't continue to pursue your dog. It's likely that he'll get caught up in the chase and go farther away than if you didn't pursue him. Animal control officers and rescue organizations have learned that dogs who are loose will generally stay within a 1 mile radius of where they got loose, unless chased further. So notify the local police, the local animal control officer, animal shelters, and near-by veterinarians and tell them that your dog is running loose. Describe him and any collar or leash attached to him. If he's not found in a few hours, put up lots of posters, with his picture (keep a jpg. photo of your dog on your computer!!); circle around the point where he got loose as you post. The first line on the poster should read REWARD! because that's what gets peoples' attention. And include your home and cell phone numbers, as well as the phone number of your animal control officer.