Diagnosis
The battery isn't taking a charge. This can happen if the battery has been deeply discharged (left dead for months) or has reached the end of its lifespan.
How to Fix It
Try a hard reset. Unplug the charging base from the wall and from the handle. Leave everything unplugged for a full hour. Then, plug only the base into the wall. Wait until you see the base's standby light. Then, place the handle on it and leave it for a full 24-hour charge without disturbing it. This long, slow charge can sometimes wake up a sleeping battery.
If That Didn't Work
The lithium-ion battery inside the handle has likely failed and needs replacement. Since the handle is sealed, this isn't a DIY fix. You'll need to reach out to AquaSonic customer support; they often have a warranty process for this.
Heads Up
For long-term storage, charge the handle to about 50% and turn it off. Storing it completely dead for months is a sure way to kill the battery for good.