Perhaps a colleague used your machine and didn't know that your are not in the habit of switching off the switch at the back. If there is, there must be some power getting to the equipment. Perhaps a separate lead to the screen has come loose. Switch the power off at the wall socket, push all leads firmly home and try again. If so check that it is on.
Users regularly fail to distinguish between power to the base unit and power to the screen.
This happens in reverse as well. Often we ask a user over the phone to power an machine down and then up again only to find out subsequently that they only powered down the screen. The machine is actually functioning but someone has turned the brightness control right down so that the screen is blank. This sometimes happens as a well meaning attempt at a screen saver or as a prank with student machines. Switch the machine off completely.