ne s'allume pas du tout

Any lights or vibrations ? Try removing the battery then holding the power button for a few minutes then reinstalling battery and powering it on . If this brings no joy then try getting into the recovery menu. This shows how to do this

https://www.youtube.com/watch?v=q23KvYy-...

You may want to just do a cache partition wipe first instead of the factory reset . You may just need a fresh boot . If that doesnt work then go with the factory reset but understand when you do it will wipe the phone clean and you will lose your data. Hope this helps