DIAGNOSIS Poor electrical connection between the vacuum's charging pins and the dock contacts. Dirt or misalignment is breaking the circuit.
FIX Unplug the dock from the wall. Wipe the two silver charging pins on the back of the vacuum with a dry cloth. Clean the two spring-loaded contacts inside the dock's cradle with a cotton swab and a tiny amount of isopropyl alcohol. Let it dry. Firmly seat the vacuum on the dock, listening for the click.
IF THAT DIDN'T WORK The dock's power supply (the wall brick) may have failed. Try a different compatible 42V Samsung charger if you have one.
HEADS UP Do not force the vacuum onto the dock. If it's not aligning, check for debris in the guide slots. Bent pins are not user-serviceable.