@sylvain_merle have you managed to get the phone started again?
If not did you install before the update something like Xposed or OpenGapps?
If you did you first need to go to Recovery Mode (TWRP) and reinstall those as well.
If this is not the case but you still have problems, even after checking the battery try again to get into Recovery Mode and install the update manually with the link @Lidwien gave in the first post.