J'ai effectivement trouvé la reponse sur le forum.
Remove the back cover.
Press and hold down the OFF button using the tip of a pen or a similar object.
The device turns off automatically.
Do not use overly sharp objects that could damage the OFF button.
If problem persists you can do a repair of the phone software via Sony PC Companion:
Click on this link to get Sony PC companion:
http://www.sonymobile.com/gb/tools/pc-companion/
Start PC companion, start "Support Zone", start "Update phone software" and click on "Repair".
Follow the on-screen instructions.
Ouf, et ca fonctionne.
