Device blocked

Hallo Everybody,

Today, after my commute I pressed the power button to put the device to sleep, as I have done many times before.

Since then, I can't seem to restart the device (or the screen).

The unit connects to my mobile phone over bluetooth and I can plug it into my PC just fine as storage medium.

When I press the power button, I can see the backlight of the screen activating, but it remains black. Because of this I think that the screen is functional.

But I can't figure out how to reset / restart the device (or screen).

Does anybody have an idea how to get the unit out of this state?