Diagnosis
The brush's Bluetooth module has likely timed out or glitched. It's a common sync issue, not a permanent failure.
How to Fix It
Force a hard reset of the brush's Bluetooth. Hold down the power button for 10-15 seconds until the brush turns on and then off again (you'll feel it vibrate). This resets the connection. Then, in your phone's Bluetooth settings, delete/"forget" any old "Philips Sonicare" devices. Now open the Sonicare app and try pairing again from scratch.
If That Didn't Work
Check for phone-specific issues. Ensure the Sonicare app has permission to use Bluetooth and Location services (Android often needs this for device discovery). Also, close all other apps and temporarily disable any VPN.
Heads Up
The brush only broadcasts its Bluetooth signal for a few minutes after you turn it on. Start the pairing process immediately after pressing the power button.