One of the most common issues why an HP OfficeJet Pro 8620 Wireless printer would go offline is because of keeping the printer in an idle state for a prolonged period. In addition, incorrect printer settings can also lead the printer to go offline. You can follow the below-mentioned steps to resolve such issues:

1. Turn off the printer and disconnect the printer’s power adapter from the electric socket.
2. Wait for two minutes and connect the power adapter back to the electric socket; turn on the printer and check if the printer is online.
3. Check if the printer is connected to your computer either wirelessly or with a cable, such as USB.
4. Go to the Control Panel and then select ‘Devices and Printers’.
5. Right-click on your printer and select ‘See what’s printing’.
6. Select the ‘Use printer Online’ option and check if the issue is resolved.