According to Apple Support:

In certain situations, the Loop Index may become unusable and may need to be rebuilt.

If this happens, you may see these symptoms:

* A "No Apple Loops Found" message appears when you open the Loop Browser.
* Loop Browser buttons are dimmed and no loops appear in the Browser.
* Loops appear in red in the Browser.
* There are other issues involving Apple Loops.

To rebuild the index files, follow these steps:

1. Log in with an administrator account.
2. Quit Logic if it is open.
3. Click the Finder icon in the Dock.
4. From the Go menu, choose Go to Folder.
5. Type the directory location of your Apple Loops. (See Where loops get installed for more information).
6. Click Go.
7. Drag all "Search Index" files from the Apple Loops Index folder to the Trash. (These files will be rebuilt later.)
8. Open Logic.
9. Click the Loop Browser button. You may see the "No Apple Loops Found" message.
10. Click the Finder icon in the Dock.
11. From the Go menu, choose Go to Folder.
12. Type the directory location of your Apple Loops. (See Where loops get installed for more information).
13. Click Go.
14. Locate the Apple Loops folder and individually drag each folder within it from the Finder to the Loop Browser in Logic.

Be sure to complete steps 11-14, checking all the possible locations for loops, as described in Where loops get installed.

The last step will index the Apple Loops folder again. If you have installed GarageBand Jam Packs, repeat the last step for the GarageBand Jam Pack folders.