Up until roughly sixty years ago, in the states, Romex had no ground. Proper protocol for that situation is to install a GFCI breaker or outlet at the start of the circuit to provide protection to the circuit that lacks grounding. In your case, I would bet that you have a connection at a receptacle box, behind a receptacle, where the ground splice had fallen apart. Not an uncommon problem, and easy to fix. Turn the circuit off, test to confirm that it's off, then start pulling receptacles out of their boxes and check splices. the first rule of electrical troubleshooting is "don't waste time in the middle". If you can visualize a flow of power here, it starts closest to the panel the finds it's way to the first outlet without a ground. Take that one apart, of the one closest to it, that appears to feed it, first and work from there.