One thing of course is to wait for the battery to run out, then connect the charger and power on the bangle. That seems like the only way for you to reboot it and hopefully it will not freeze again. Maybe don't charge it too much in case you have to do the same thing again.
Then when it's up and running I would try what @Mi suggest regarding apps and firmware via the app loader.
Maybe you can find some useful tips from @Gordon in this thread as well.
Sorry, forgot to mention that we tried to charge multiple times.
I even warmed up the watch to around 40 degrees before attaching the charger once. Also tried multiple charging devices. Current between both charger pins is 5V.