Try doing a hard reset: http://support.apple.com/kb/HT1430?viewlocale=fr_FR <==== c'est en Français!
If that doesn't solve the issue, you could also try backing up the device and restoring the device as new.
In that case, follow the steps in: http://support.apple.com/kb/HT4137?viewlocale=fr_FR <==== c'est en Français!
NB: set up as a NEW device because a software issue will be in the backup. If the issue persists after restoring as new, you should offer the device for service.
If you're not able to restore the device, this one might help: http://support.apple.com/kb/HT1808?viewlocale=fr_FR <==== c'est en Français!
Using his article will erase everything on the device, so make sure you have a backup!
Bon chance !