Did you try a long press (press button for ~10sec).
Release immediately after you see the "watchdog" screen - before the <==== appear (if you are too late, no problem - just repeat).
That should reboot the device.
If you run into the same problem again, you may check the "more" tab at https://banglejs.com/apps/ . There you can remove all apps and install default apps.
You can always install apps again later after you see hardware running ok with the basic apps.
If you search for "firmware" in the library tab at above link you can also find the firmware updater there.