Where are your song? On an external drive or your system drive?
Do you remember, when this started? Did you install anything new at that time? Audio plug-ins? Update software? Did you run cleaner applications, like MacKeeper?
Justs in case this is a problem of failing hardware, is your backup of your system current? This may be a problem of a failing drive.
Try to narrow down the problem by making a few tests.
- Can you save and open GarageBand projects on a different drive? For example thumbsdrive?
- Can you open and save GarageBand projects when working from a different user account, see: Isolating an issue by using another user account
- Can you open and save GarageBand projects when working in Safe Mode? See: Mac OS X: What is Safe Boot, Safe Mode? and Mac OS X: Starting up in Safe Mode
Please report back, when you know, if a different drive, a different account, or safe boot mode change anything.