Diagnosis
The printer's toner level detection is based on a page count estimate, not an actual sensor. It's just hit its programmed limit and needs a manual reset.
How to Fix It
Open the front cover. You'll see the orange alarm light is on. Press and hold the 'Stop' button on the printer's control panel for about 5-10 seconds. The light should go off. Close the front cover. The message should clear and it'll operate normally. You can usually get a few hundred more pages out of a 'dead' cartridge this way.
If That Didn't Work
If holding the stop button does nothing, the cartridge chip itself might be locked. Try taking the toner cartridge out, gently cleaning the gold-colored electrical contacts on the cartridge and inside the printer with a dry cloth, and re-seating it firmly.
Heads Up
This reset trick works on the standard Canon 054 toner cartridges. If you're using a third-party or remanufactured cartridge, its chip might not support the reset, and you'll just have to replace it.