When I was doing computer repair for a retail store I got a call from a school district that we servied.
They said their monitor was not working.
I asked if they had checked all the connections and they said yes.
I drove to their office and walked in. From across the room I could see that the power cord on the monitor was hanging loose under the desk. Not plugged into anything.
I plugged the cord into the power strip under the desk, the monitor came to life, and I left without comment.