Diagnosis
The paper presence sensor (a small metal lever inside the tray) isn't being engaged, usually because the paper stack isn't pushing it up or the stack guides are too loose.
How to Fix It
Remove the paper stack. Reseat it firmly against the right-side guide, then adjust the left guide to snugly touch the paper stack—you should feel slight resistance when sliding it. Push the tray fully in until it clicks. The blinking light should turn steady.
If That Didn't Work
The sensor lever inside the printer (at the back-left of the tray cavity) might be stuck. Power off, open the tray, and use a flashlight to look for a small plastic lever. Gently move it up and down a few times to free it.
Heads Up
Loading paper above the "max fill" line (about 250 sheets) can bend the sensor mechanism, causing permanent false empty errors.