@marathon2422 That's it! By using the BQ flash tool i could reawake my device.

Unfortunaltly i had to flash my device with am android package cause BQ don't provide an ubuntu package for the HD edition anymore. Afterwards i could reflash the device back to UT by using the UBPorts installer!
Now i'm glad to have a running device and i stricktly avoid to power it down...

Btw: after reflashing to android the battery power was only 1%. This after shutdown only 48h before. So the shutdown was defintly incomplete and the device drained the battery strongly.
My question now: is this a BQ problem? Or is it a UT bug?

Thanks to all for giving me tips and advices! Now i can restart with UT