DIAGNOSIS The printer's internal waste ink pad has reached a pre-set limit. It's a safety feature to prevent leaks, but the printer will lock up until it's reset.
FIX You'll need to do a service tool reset. First, download the official 'Service Tool' software for Canon printers. With your printer turned on, connect it to your computer with a USB cable. Run the tool, select your printer model, and look for the 'Ink Absorber' or 'Waste Ink Counter' reset function. Follow the on-screen prompts. The printer will restart and should be clear.
IF THAT DIDN'T WORK If the software doesn't connect or the reset fails, the physical pad might be genuinely saturated. You'll likely need to contact Canon support or a repair shop to have the waste ink absorber pad inspected and possibly replaced before a reset will work.
HEADS UP This reset is a maintenance function, not a fix for a leak. If you see actual ink pooling inside the printer, stop using it immediately and unplug it—you've got a different, more serious problem.