@kte This is an English speaking forum. Please use some kind of online translator. (Ceci est un forum anglophone. Veuillez utiliser une sorte de traducteur en ligne.)
We need more information to be able to help:
Which version of FOG did you install?
Any errors when the installer was running?
Is this a clean Ubuntu install or some server you already had in production and reused for FOG?
Can you connect to the database on the command line? - SSH to your server and try mysql -u root -p - are you able to login?
Check /var/www/fog/lib/fog/config.class.php - DB settings defined as DATABASE_USERNAME, DATABASE_PASSWORD and so on.
