How much battery did the device have? the only time i have had similar (I'm on an OSTC Plus so not quite the same as this has a much smaller battery) is because the battery had not been used for a while so needed a bit of a recharge to boost this back up again.
- Connect to USB
-Toggle a reset with a (quite strong) magnet at the center of the screen
If the unit wakes up, make sure to recharge the unit (1) until the battery sign is green. All this assumes you have the 3.15 installed already. If not: Do so (2).