Diagnosis
The printer's internal page counter has decided the black toner cartridge is 'empty' based on its estimated yield, not an actual physical sensor. It'll eventually refuse to print.
How to Fix It
The official fix is to replace the black TN-338 toner cartridge. But first, try resetting the toner counter. Turn the printer on. Open the front cover, then close it. Wait for the 'Toner Life End' message to reappear on the screen. Quickly press the 'OK' button. The screen will ask 'Replace Toner?' Select 'Yes'. The message should clear for a while.
If That Didn't Work
If the reset trick doesn't stick, there's a chance the chip on the toner cartridge itself has permanently locked. Your only option then is a new TN-338 cartridge.
Heads Up
You can often get quite a few more pages out of a 'life end' cartridge, but print quality will eventually fade. Don't shake the cartridge violently to extend life—you can damage the internal mechanism and cause leaks.