sudden crash of ubuntu phone Aquaris 4,5, can not start it again, all black screen. what to do?

after recharging the battery to full and after installing a app/scope (ch.ch) I wanted to try uninstall a game (cut the rope), the game opened, I stoped it with a finger wish left right, then with a fingermove right-left I removed the tab of the game; after I wanted to go on the ubuntu store again and it went all black. Now I can not restart it again, there are no sign of life.
What can be done?
Thanks a lot, Urs