When I handle support problems like this where unplugged cables are likely, I make the user triple check everything is plugged in and bet a coffee on it. I've got it a couple times and I feel it makes people actually check their cables!
I used to tell them that what they should do is unplug the cable, and plug it back in again but the other way round -- in other words, with the ends reversed. From end A in port 1 and end B in port 2, to A/2 + B/1.
This, of course, forced them to check both ends and in the process made them ensure it was the same cable.
I made up some BS about corrosion patterns on the connectors to justify it, but it worked often enough to keep using it. Sir Terry Pratchett might have called it "headology".