I've had this exact thing happen before, one of those " we can't pull this loaded trailer home from an hour away" moments
the fix was so stupid that I still can't believe it worked, I had someone get the truck up to running speed, and I yanked out the breakaway cable, plugged the cable back in and its never stopped working since
not sure if it was the sudden jolt that rattled something or a power dump from the battery, no idea but as far as I'm aware we've had zero issues since
controllers get power from your brake light switch, when you hit the TBC the truck brake lights should come on, if not, and the controller is still connected properly, I'd say its a bad ground or bad controller
its a weird system IMO, but works, I won't say it works well, but it works
my dad runs a "smart" controller that is supposed to self modulate the gain and add brakes going downhill, it never works right, either you get no brakes or it locks up, and the lever on the controller can't over ride the "max" brakes that you set in the programming
I put a dumb controller in my dump truck that has a delay where it slowly ramps up to a preset power, no more and no less, and the manual lever over rides the entire system, I like it a lot more, especially after that one time going through an intersection a few years ago and my truck became the trailer