Keep holding the button, and release the Stop/Reset button.

Printers are built with internal waste ink pads (absorbers) that collect excess ink during initial setup, printhead cleaning cycles, and borderless printing. To prevent ink from leaking out of the machine and ruining your furniture, Canon programs an internal electronic counter. Once this counter reaches 100%, the printer locks itself down for safety.

While still holding , press the Stop/Reset button 5 times (some models require 6 times). Release both buttons simultaneously.