Disconnect the live feed and switch return to your switch at the loop in junction box. With the switch in the closed position (ie light switch turned on), set your multimeter on resistance/Ω and measure for continuity.I know, which is why I am stumped. My only deduction is that the problem is with the connection of the switch feed to the loop. So I will need to trace this back to where it links in.
This will confirm if you have a break in your switch wire - there will be no continuity.