Hello. Try this:
1. Start a new project in GarageBand. Save it and quit GarageBand.
2. Go to your corrupted project, the one that won't open and do a 'apple right click' (control + click). Select Show Package Contents.
3. You will see two files in your project, a media folder and projectData. Open projectData in an editor (textEdit). In this file you will see a bunch of xml fields. The biggest entry is in the field <data>. Copy everything from the data field (no xml flags - nothing surrounded by < or >). It will be a long list of weird characters.
4. Go to your new project that you just created and saved. Show Package Contents for this file. Open the new project's projectData.
5. Paste the <data> from your corrupted project into the <data> of your new project.
6. Copy your media project from your old project to your new project.
7. Close finder window.
8. Double Click your new project and GarageBand should bring up your music.
Hope that helps!