I'm sorry you are having issues with Garageband. The troubleshooting steps outlined in the article below should help you on a path to resolution ....
Delete the User Preferences
You can resolve many issues by restoring an iLife application back to its original settings (this will not impact your project files or libraries). To do this:
- In the Finder, click the Go menu and choose Home.
- Open the Library folder and then the Preferences folder.
- Remove the "com.apple.x.plist" file, where "x" should be replaced with the iLife application you are troubleshooting (for example, "com.apple.iphoto.plist" or "com.apple.imovie8.plist").
Remove receipts and reinstall iLife applicationsFor Mac OS X version 10.5 Leopard:
- In the Finder, choose Applications from Go menu.
- Drag the application you are having issues with to the Trash.
- Go to your internal hard drive and open Library folder. Then, open the Receipts folder. Locate any receipts with names that begin with the iLife application you are having issues with; move these files to the Trash.
- Insert your iLife installation disc and install only the iLife application you previously dragged to the Trash. (You can select and install individual applications by clicking the Customize option under Installation Type).
- When finished, choose Software Update from the Apple () menu to update your software to the latest version.
I hope this helps ...