Hi, Polianalliescu,
forgive me for asking basic questions, but has indeed your project file been created with a much older version of MS Project (2000 or prior)? If so, that could be the reason, as newer versions of MS Project cannot open file formats prior to those created before (and up until) the year 2000 [version of MS Project]. Other than that, I would like to know more details about your problem in order to help you more (current version of MS Project, project data file format, etc.).
I hope this helps.