DIAGNOSIS The paper feed sensors are dirty or the paper isn't sitting correctly against the pickup roller. Dust and paper debris build up and trick the printer into thinking the tray is empty.
FIX Take all the paper out of the tray. Use a can of compressed air to blow into the tray slot and the area just inside where the paper feeds. Wipe the little rubber pickup roller you can see inside the feed area with a lint-free cloth dampened with a little water. Reload the paper stack, make sure it's under the metal or plastic guide, and firmly tap the stack on a flat surface to even it out before reinserting.
IF THAT DIDN'T WORK The paper type setting in your print driver might be wrong. On your computer, go to Printer Properties > Main tab and make sure 'Plain Paper' is selected for the Media Type, not something like 'Envelope' or 'Heavyweight'.
HEADS UP Don't overfill the tray past the line marked inside it. Also, avoid using paper that's been stored somewhere humid, as it can stick together and cause feed issues.