Garageband (virus?)

So, I'm new at this... I just got my Mac about a year ago. and have been using Garageband alot. I work really hard recording and composing music and then every now and then I'll get this 'file not found' message when I go into my composition file. So then I lose a whole section of my piece of music. It seems to be like a virus that slowly eats away parts of my music, here and there. continually. What is this? (does anyone else get this) and how to stop it?
Thanks