This could help:
https://linux-sunxi.org/PocketBook_Touch_Lux_2_(626)
Especially:
Quote:
Entering FEL mode with default pocketbook software: device powered off, press right button (square symbol), power on while keeping the button pressed. There is not LED or indicator showing if the device is on. However plugging the device to a computer through USB will show there is something. lsusb:
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Could it be that one of your buttons got stuck?
And some more infos here:
https://linux-sunxi.org/FEL