HP Officejet Pro 8600 Plus shows the following error on the LCD screen and is completely non-operational.
There is a problem with the printer or ink system. Turn printer off, then on. If problem persists, contact HP.
the issue was caused by a refilled ink cartridge. It seems that the cartridge had run out of ink, but the chip was "telling" printer that there is still some ink left...
Replacing the offending cartridge resolved the issue.