PS3 reading game as a movie disk?

Hi.

I was hoping that you could suggest ways to fix the problem that I'm having...

After a month I come back to playing Final Fantasy XIII on ps3. Put the disk in and it reads it as a video disk...

Goes straight to the video section and tells me that it can't be played. No option at all in the game drop down menu in home to play it.

Help would be very appreciated. It's my favourite game series growing up and it was my brother's game.