Factory crimp connections can be so poorly done, the wires slip out of the connectors by just touching them (BTDT). And the crimp tool may have been used on the plastic cover and not the metal barrel inside on a compression connector/terminal (BTDT too). And splices using marrettes my have also been done poorly. So when you work with or near something 12 volts, you can cause wires to separate. I would say the culprit connection is in the vicinity of where you were working and something you touched.
Lights, fans, etc. are wired in series from one to the next so somewhere in that circuit you've caused a connection to come apart. If you've taken the voltmeter and verified the presence of 12 volts where you are working, remove 12 volt fuses until the 12 volts you've got turns off. Then put the fuse back in and remove all others. See what 12 volt lights are still on and pull out nearest one(s) and check the connection there to see if a connection has come apart. If problem not found, try gently wiggling wires in the ceiling to see if you can tell where a wire (2 conductor) goes to a light in the bathroom and then check the connections there.
12 volt wires are simply laid on top of the bottom chords of trusses and run like sloppy spaghetti all over. Lots of slack to wiggle/tug on wires. You can also get access into the ceiling space through lights, AC grilles, skylight bezels, etc. and you can get a hand up in there to see which direction wires run.
Also, if you look at the wires, there may be a color coding stripe along them which will help identify what wire is what. There should be no need to install another ground wire. Wires will run down inside walls to get to the panel and are also run in the hollow space under cabinets. Splices can be hiding in all sorts of places.
