Really, are you sure you checked?
If you help people for money or free, it’s often the first question to ask. It’s also one that people Will Lie About. Well, maybe not lie, but presume about, gloss over, fail to check carefully, not take seriously.
Happens all the time. That’s why I often tell people to unplug and replug cables, rather than merely asking are they plugged in properly. Network cables especially lend themself to that.
So please, if you’re the hapless computer user with the problem, please pay attention to the basics like this. Never assume. Never brush it off.