Back in the day keyboard and mouse were connected to the back of the computer with identical (but color coded) PS-2 plugs.
Today one would expect that swapping two mechanically identical plugs would result in no major trouble - think USB - but back then COLOR CODES HAD TO BE OBEYED!
Or computer science teacher came to help when our PC won’t boot up, tried reset, tried reset again, fiddled with the BIOS, gave up.
That wasted more than halve the lesson’s time just for us to swap plugs back to working while he wasn’t watching.
Sometimes things simply don’t work when the teacher is around…
Aaaand thats why you have to start with the basics when troubleshooting. Even if no one has fucked around with the machine, make sure everything is connected properly.
