According to the official HP support website for the HP Deskjet 840C, resetting the printer is also known as performing a power reset. A power reset drains all of the power from the Deskjet 840C, and then resets the printer after it is restarted. Furthermore, the official HP support website indicates that a power reset can be used as a troubleshooting step to solve several problems including a stall in the printer software installation process.
Press the Power button on the HP Deskjet 840C to turn off the printer. The Power button is located on the front, left panel of the printer.
Remove the power cord from the back of the printer.
Unplug the other end of the power cord from the power source and wait at least 30 seconds. This allows the power to completely drain from the 840C printer.
Plug the power cord back into the outlet.
Insert the power cord into the back of the 840C.
Press the Power button to turn the HP Deskjet 840C back on. You have reset the printer.