The printer stops printing due to outdated firmware and depleted ink cartridges. Power off the printer and halt until the printer turns idle. Disable the printer’s power connection by unplugging the power cable. Disconnect the printer from the wireless or USB connection. Restart the router or modem (the one you use). Power on the 123.hp.com/ojpro8720 printer and tap the wireless button on it. The printer connects to the available network.
Source: https://123-hp.chat/hp-officejet-pro-8720-troubleshooting/