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.