As your headphones aren't turning on, even though they are showing as fully charged, what I would recommend you do is a reset of the headphones. To do this what you will want to do is:
Power off your headphones.
Wait 30 seconds.
Connect your headphones to a wall outlet using a USB charger.
Wait five seconds.
Disconnect the USB cable from the headset.
Wait one minute.
Power on your headphones.
If you are still unable to turn them on after doing this, what you will want to do next is reach out to your local support team for further assistance. You can find the contact information at this link, by selecting your country/region and then the 'Contact Us' option at the bottom of the page.