DIAGNOSIS The built-in descale reminder has been triggered and locked the brewer. It requires you to complete a full descale procedure before normal operation resumes.
FIX You must run the full descale cycle. Get a Keurig descaling solution or a citric acid/vinegar alternative. Empty the water reservoir. Fill it with the descaling solution mixed with water. Initiate descale mode by pressing the Menu button, navigating to 'Descale', and confirming. Follow the on-screen prompts—it will cycle the solution through, then prompt you to rinse with multiple tanks of fresh water.
IF THAT DIDN'T WORK If the machine locks up during the rinse cycles, it may have a temporary fault. Unplug it for 5 full minutes to do a hard reset, then plug it back in and immediately restart the descale process from the menu.
HEADS UP Do not try to bypass this by unplugging it—the reminder is stored in memory. Using a non-Keurig descaling solution is fine, but you must do the full rinse cycles or your next several coffees will taste awful.