Hello everyone and sorry for my bad english.
Ok, this was my problem :
I created my project in Prelude CS6 then worked out in Premier Pro CS6 and then send it to SpeedGrade CS6. After closing everything for a reboot, my main project Premiere couldn't be opened, error message : " This project contained a sequence that could not be opened. No sequence preview preset file or codec could be associated with this sequence type. "
So, after looking on all those forums I decided to reinstall all my CS6 programs with Adobe Creative Suite Cleaner. Somehow, the cleaner doesn't clean everything, 5 of my CS6 programs were still there. And when I tried to reopen my project with the fresh Premier CS6, the same error message shown his face.
This is my solution :
Open a new Premier project and import your "work" project in the new one. When Premier ask you how to import, choose Import selected sequences. This worked for me.
( Any of my CS6 were updated ).
Also, my " old " project had dynamic link with After Effect CS6, maybe all this cocktail made this error.