DIAGNOSIS: The battery is either completely dead and cannot hold a charge, or there is a poor connection between the robot and the charging dock contacts.
FIX: First, clean the metal charging contacts on the bottom of the robot and the front of the dock with a dry microfiber cloth. Then, place the robot on the dock and ensure the blue charging light illuminates. Leave it for 4+ hours.
IF THAT DIDN'T WORK: The battery may need to be 'jump-started'. Remove the robot from the dock, hold down the POWER button for 10+ seconds, then place it back on the dock. If it still won't charge, the battery is likely dead and needs replacement.
HEADS UP: Lithium batteries degrade if left completely drained for long periods. If you're storing it, leave it on the dock. A replacement battery is a standard part you can install yourself in minutes.