Fire Emblem Disc stops working...?

I put in my Fire Emblem Radiant Dawn (Wii) disc in my Wii and I beat the final chapter of the first part. After I do this the game stops reading. I've heard of glitches based on the difficulty one is playing on, but I haven't heard of this. If it matters there is a very small scratch on the disc. I tried the cleaning method and it didn't work. If some one can tell me how to fix this it would be appreciated.