DIAGNOSIS The paper pickup rollers inside the printer are likely dusty or worn smooth, so they can't grab the top sheet of paper from the stack to feed it in.
FIX First, remove all the paper from the tray. Fan the stack like a deck of cards to separate the sheets, then reload it, making sure the stack is under the maximum fill line and the paper guides are snug against the edges. If it still fails, you need to clean the rollers. 1) Unplug the printer. 2) Look inside the paper feed area at the back of the tray slot. You'll see a gray rubber roller. 3) Dampen a lint-free cloth with a little water (no cleaners!), and wipe the roller while rotating it with your fingers to clean its entire surface. Let it dry completely, reload paper, and try again.
IF THAT DIDN'T WORK The paper sensor, a small plastic lever inside the feed path, might be stuck. With the printer unplugged, use a flashlight to look into the feed path from the tray. You might see a small plastic flap. Gently move it up and down with a pen to make sure it's not jammed in place.
HEADS UP Paper that's been in the tray for months in a humid or dusty environment can get a slight 'curl' or a coating of dust that makes it slippery. Always use a fresh, crisp ream of paper for troubleshooting feed issues.