A friend of mine had that happen to a server once. Someone unplugged it as they thought it had no purpose and would save on the electricity bill by having it turned off.
They assumed a computer with no-one sitting at it couldn't possibly be in use!!
Later, the machine was set up with a UPS (uninteruptable power supply) which is a little machine that sits next to the server, acting as battery backup should power fail (or be turned off!). Both had big 'do not switch off / unplug' signs attached.
The aforementioned person happened by again, saw the signs but examined the machines anyway. Apparently there was a switch on the back that said "240v - 110v". Logically(or so this person thought), it would cost less to run at the lower setting, so the switch was duly flicked. The ensuing 'bang' and smell of burning plastic lead to a huge bill for a new UPS and that person being banned from the building........