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This is pretty common in the game. If your Sim reads the newspaper regularly, you'll find this happening periodically. There's no way to bring them back to town through regular game play. Even if you go back to a previous save, you'd have to act quickly with whatever you hope to do with that family because they will probably leave town again. Some things like that are just programmed into that particular game file and will happen unless you take major steps to move the family into your house or something similar.
I once had a multi-generation family where I moved the adult children out after getting them married and pregnant. One of them lived right next door and moved out of town! Because they were family, I could still invite the adult child over to visit. I did some maneuvering... invited the adult child to move in, then had him invite his family over and moved them in. Next, I moved them back out again into the same house next door. However, the difference here is that they were family and showed up on my relationship panel. Without that, you're just stuck without that family in town.