DIAGNOSIS The printer isn't sensing the paper. This is usually because the paper stack is too high, the paper type setting is wrong, or the little metal paper-present sensor isn't being triggered.
FIX 1) Take all the paper out of the tray. 2) Flex the stack (fan it) to separate the sheets and remove any static. 3) Reload the paper, but do not fill it past the "max" line indicated inside the tray. Overfilling is a huge cause of this. 4) Make sure the paper width guide is snug against the stack—it shouldn't be tight enough to bend the paper, but it shouldn't be loose either. 5) On the printer's control panel or in your computer's print settings, make sure the paper size (like Letter or A4) matches what you actually loaded.
IF THAT DIDN'T WORK There might be a small piece of paper or debris stuck in the paper pickup roller area under the tray. Remove the tray and use a flashlight to look inside. You can gently wipe the rubber rollers with a damp lint-free cloth if they look dusty or shiny (slippery).
HEADS UP Using paper that's too heavy or thick (like cardstock) in the main tray can cause this. For those materials, you're better off feeding a single sheet through feed slot in the back.