I had a Focus I was selling. Guy came to look at it and I assured him everything on the car worked, because it did.....until he got there to look at it. He was trying out all the power windows when BAM!!!! The left rear one was half way up and just quit.....it stopped.....the motor went bad in an instant like that.
You need to somehow check voltage to the motor when you press the switch, or, just turn your key on, hit the switch, and watch your dome light....if it dims slightly or kinda flickers when you press the window switch, that means power is flowing but the window motor won't run, and you have your diagnosis. It's a hillbilly way to diagnose, but it works.
If you were in there doing speakers though, it could be something you missed if it worked before.