Expanding on what Lee is saying - if it's all wired properly as a ring, you have to have 2 separate breaks in either live or neutral, (or one of each) to cause more than one socket to stop working. By 'Ring' we mean that the cable goes from the distibution box, round all the sockets and back to the dis box again, so a single break won't stop the ring working.
If however the extension is wired as a single-ended spur from the main ring, then a single break will stop everything after it from working.
You're going to have to start taking sockets off to see if all the wires are secure - every socket should have pairs of live and neutral in them if it's wired as a full ring. If you find one with three pairs, that indicates that there is a spur wired off somewhere.