DIAGNOSIS This usually means the printer has lost its connection to your Wi-Fi network. The little wireless light on the printer is probably blinking or off.
FIX First, let's get it back on the network. 1) On the printer's control panel, press the Wireless button (looks like a dot with curved lines). 2) Use the arrow buttons to scroll and select "Network" then "Restore Network Settings." Confirm. The printer will restart. 3) After it comes back on, you'll need to reconnect it. Hold down the Wireless button for 3 seconds until the light blinks, then go to your computer and run the HP Smart app. It should find the printer and guide you through setting it up on your Wi-Fi again.
IF THAT DIDN'T WORK The connection might be blocked. Turn off your router and printer for 30 seconds. Turn the router on first, wait a minute, then turn the printer on. This often clears up the handshake between them.
HEADS UP If you've changed your Wi-Fi password recently, the printer won't know the new one. You'll absolutely have to go through the reconnection steps in the HP Smart app.