Okay, first my thanks to all and some new ideas and to see that my son is not allone. He came yesterday with his new tablet (Archos 80 G9) and was sad because it didn't restart.
So I try some things, read the documentation (it is different discribed in different languages!) but nothing helps.
When I plug it on the power charger, the green light binks-blinks-pause for some time. I reset and plug it again. Same but sometimes the led light but later it goes out. Until now the tablet didn't boot. No Archos logo...
So I will try:
- another button-charger-button sessions
- plug to a PC
- boot into bootloader
- warm the device up (hair dryer)
Hopefully I can make my son happy...