Diagnosis
Water has gotten inside the handle and is causing a short circuit, tricking the power button. This is a common issue if the seal around the button is compromised.
How to Fix It
First, take it off the charger. Press and hold the power button for 30 full seconds. This can sometimes reset the internal circuit. Then, leave it off the charger in a dry, warm place (like on a towel near a vent) for 24-48 hours to let any internal moisture evaporate. After that, try charging it again.
If That Didn't Work
The internal damage might be permanent. Before you give up, ensure you're not accidentally pressing the button when you place it on the charger. The button is sensitive, and a slight misalignment on the base can trigger it.
Heads Up
Avoid pointing the shower stream directly at the handle and never submerge it. The Aquasonic is water-resistant, not waterproof, and the seal around the button is the weak point.