Had to figure out why an instructor here at the community college where I work couldn't log into the front teacher station in the particular lab he was in. The whole machine just seemed frozen; you couldn't even Ctrl-Alt-Delete.
Then I saw the problem in its true, raw, naked ugliness: The keyboard was jammed!!!
Yes, hard as it may be to believe, but it does happen, so remember kiddies, always check that ridiculous tray that sticks out where the keyboard sits on to see if somebody carelessly shoved it back into the desk as they left in a hurry, thereby causing F1 - F12 to be hopelessly squashed and preventing any kind of interaction with the computer whatsoever.
It's not pretty, but we have to deal with it.
Boy, I love working tech support!