Maybe check the configs on the kit plugged into it?
In the face of an 'inaccessible switch, no known config, mission critical' situation am I the only one to think of the bleedin' obvious? As in: check the configs of all the *other* devices plugged into that switch and thus just reverse-engineer what it was doing? Switches don't work in isolation. It's not that hard. Been there, done that. Doing it again at the moment, as there's one on a network I've just 'inherited' :-(