Pon. If its old style wiring and fuse box its highly likely to be a radial cicuit. This is like one big spur off the fuse box. One for upstairs one for down. If this is the case it will be more hard to find the culprit socket!. You need to start removing screws and pulling away the sockets starting in the extension. This is the only way you will find the loose/broken wire without a tester. Saying that, the neutral wire may have also possibly come out of the junction box that has been used to spur off to the extension. But, its more liely to be a socket as if the screw hasnt been quite tightened enough the tension on the wires when crammed into the back box eventually come out!

HTH