I'll guess that he didn't mean the brake disconnect lanyard, but rather, the hot lead from the battery going to the switch. (the output of course, going to the electric brakes)
That wire wouldn't just become disconnected without some force, such as getting caught in the tongue jack.
It is probably at a junction point between the battery and the converter. It got pulled apart, and hasn't been put back together correctly, i.e. no battery connection to the trailer.
Of course, this is just a guess. Without knowing the model, and the use of ambiguous terms make this hard to offer any help.
Marty and Martha
2015 Z-1 211RD
2012 Nissan Titan