It's worth checking your connections at the back of the box. It's possible one of the other users accidentally unplugged the mouse/keyboard and plugged them back into the wrong sockets - I would be amazed if this was the case as you'd need to be colour blind to do this!!!
The "over ride" box in the center of the screen - is it anything like what you'd get if you powered on the monitor and not the base unit?
And the beeping - if the keyboard is plugged in correctly turn it upside down and shake it senseless - you'll be amazed what might fall out - paper clips etc....
If none of these steps help, tell us if the configuration has recently changed - has any of the other users changed the configuration without you knowing? It's also worth having a password on your BIOS to stop other messing around with it.
Good luck.
Oh, I'm sorry. Did I accidentally blow your head off.......again?