Diagnosis
The issue is almost always in the power connection, either at the wall or inside the mop's battery compartment.
How to Fix It
Start with the battery. 1) Remove the tank to access the battery compartment on the mop's back. 2) Take out the battery pack. 3) Check the metal terminals on both the battery and the mop for corrosion (white/green crust). Clean them with a dry cloth if needed. 4) Reinsert the battery firmly until it clicks. 5) Plug the charger directly into the mop (not the wall first) and wait for a solid red light.
If That Didn't Work
Try a different outlet and different charger cable if you have one. The problem could be a failed charger.
Heads Up
If the mop was stored in a very cold or damp place like a garage, let it warm up to room temperature before troubleshooting. Electronics can act up when chilled.