DIAGNOSIS Your printer has paper loaded in the Multi-Purpose Tray (the little slot on the front) instead of the main paper cassette. It's waiting for you to confirm you want to use that sheet.
FIX This is easy. Look at the control panel. It's asking you to press either the Start key (the green circular arrow) or the OK key to proceed with feed. If you don't want to use that tray, just remove the paper from the Multi-Purpose Tray, close the tray flap, and put your paper in the main paper cassette. The message should clear and it'll go back to normal.
IF THAT DIDN'T WORK The sensor flag for the Multi-Purpose Tray might be stuck. With the printer off, open and close that front tray a few times firmly, and look inside the slot for any torn paper or obstruction. A quick visual clean with a flashlight can help.
HEADS UP If you leave a single sheet of paper or even a sticky note in the Multi-Purpose Tray, the printer will detect it every time it wakes up and give you this message. Keep that tray empty unless you're actively using it for special paper.