Diagnosis
The vacuum is not making electrical contact with the dock. The dock's green light means it has power, but the connection to the vacuum is faulty.
How to Fix It
Remove the vacuum from the dock. Examine the four circular metal charging pins on the bottom of the vacuum. Clean them with a cotton swab dipped in isopropyl alcohol. Clean the corresponding contact pads on the dock. Ensure the vacuum is centered on the dock when you replace it.
If That Didn't Work
The battery may have entered a deep discharge state. Leave the vacuum on the dock, untouched, for a full 24 hours. This allows the trickle charge to potentially revive the battery management system.
Heads Up
If the battery is completely depleted after 24 hours on a verified clean dock, the battery pack itself may have failed and requires replacement.