Okay, I'm about to reveal one of the tricks of Product Support.
Sometimes you're on the phone with somebody and you suspect that the problem is something as simple as forgetting to plug it in, or that the cable was plugged into the wrong port. This is easy to do with those PS/2 connectors that fit both a keyboard and a mouse plug, or with network cables that can fit both into the upstream and downstream ports on a router.
Here's the trick: Don't ask "Are you sure it's plugged in correctly?"
If you do this, they will get all insulted and say indignantly, "Of course it is! Do I look like an idiot?" without actually checking.
Instead, say "Okay, sometimes the connection gets a little dusty and the connection gets weak. Could you unplug the connector, blow into it to get the dust out, then plug it back in?"
They will then crawl under the desk, find that they forgot to plug it in (or plugged it into the wrong port), blow out the dust, plug it in, and reply, "Um, yeah, that fixed it, thanks."
(Or if the problem was that it was plugged into the wrong port, then the act of unplugging it and blowing into the connector takes their eyes off the port. Then when they go to plug it in, they will look carefully and get it right the second time because they're paying attention.)
Customer saves face, you close a support case, everybody wins.
Corollary: Instead of asking "Are you sure it's turned on?", ask them to turn it off and back on.