Welcome to the Apple Support Communities, and thank you for the details of the issue you are experiencing with the software instruments in GarageBand.
First, I recommend starting with the steps in the following article:
If you still do not have access to the software instruments, I recommend reinstalling GarageBand. Use these steps to uninstall GarageBand:
- To uninstall apps you purchased from the Mac App Store, hold down an app’s icon in Launchpad until all the icons begin to jiggle. Click an app’s delete button (x).If an icon doesn’t have a delete button, it can’t be uninstalled. For example, apps that come with OS X can’t be uninstalled.
- To uninstall other apps, drag the app to the Trash (located at the end of the Dock), then choose Finder > Empty Trash.If you change your mind, before emptying the Trash, select the app in the Trash, then choose File > Put Back.
Then, you can redownload GarageBand from the Purchases pane of the Mac App Store.