First confirm that the charger is getting power from the wall. If None of the bands or the hub are charging, check to make sure that the charger is plugged into the wall outlet and that the power cord is firmly inserted into the charger.
If the bands are not charging, check for chemical build up on the bands or the charger and see article: My Bands are not Charging.
From time to time, the screws on the bottom of the hub may loosen. Using a Phillips-head screwdriver, ensure that the screws are tight, using caution to not over-tighten and damage the plastic.
If the hub is still not charging, contact customer service at firstname.lastname@example.org