Diagnosis
It's frozen. This is usually a software/firmware glitch, not a hardware failure. A background update might have stalled, or it's stuck in a low-power state.
How to Fix It
Perform a hard reset. Unplug the power cord from the back of the printer. Also, unplug it from the wall outlet. Wait for 60 seconds. While it's unplugged, press and hold the Power button on the printer for about 15 seconds (this drains any residual power). Then, plug everything back in and press the power button normally.
If That Didn't Work
Check for a failed firmware update. After the hard reset, if it boots but seems stuck trying to update, you might need to connect it to your computer via USB and use the HP Support Assistant software to manually re-install the firmware or recover it.
Heads Up
Avoid using a smart plug or power strip with a switch to turn the printer 'off'—it needs to go through its own shutdown routine. Always use the power button first, then you can cut power at the strip if needed for a long absence.