You can restart the doorbell by powering down and powering up the doorbell again.
To power down the doorbell, press the "Power" button on the back of the device until you hear 'powering off' from the doorbell.
To power up the doorbell, press the “Power” button on the back of the device until you hear "Hello, Wuuk" from the doorbell.
The actual "Power" button is covered by a white rubbery piece along with the charging port. The rubbery piece is to prevent the charging port and button from weather elements, but sometimes when it is not pushed hard, the button end could have a gap and is not touching the real power button.
In this case, if you have charged the doorbell, but could not get the doorbell powered on, you may want to remove the white rubbery piece and directly push on the black dot next to the charging port using a pin.