It's probably a loose or broken connection. Many times, one circuit will feed multiple receptacles in a daisy chain. Break that chain somewhere, and those downstream stop working. So, looking at those receptacles, if you find one that only has one cable coming into it that doesn't work, it is the end of that chain...you have to work your way backwards and find those with two (or more) cables coming into the box, and see if there is power on one set, but none going out (probably because of either a loose connection, or a broken wire). There are tools that inject a signal into the wiring that you can trace, but baring purchasing one of those, you have to think logically on how the things were probably wired - most likely from one next to it, or directly above or below.