Could be a socket terminal tightened up on to conductor insulation instead of on the copper core.
If your unlucky the opencircuit might be between socket outlets.
first thing you should find out is if live, neutral and earth are open. if yes, then likely a cut cable, not connected or all terminals came loose. if just 1 core, then look for loose cable. if nothin, it could be tightened onto the insulation
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