How did you reinstall your OS? Did you wipe your drive completely?
And how did you restore your documents and data? If you used Setup Assistent directly when you reinstalled, and restored your Applications and data from a TimeMachine backup, your Instrument Library and Loops should have been restored. What is your GarageBand version?
First, I do not use Time Machine. I use Carbonite. As I recall, the Apple Tech I was talking to on the phone led me through the process of dragging folders and files I might need from the MacBook Pro to an external HD before wiping. Once the OS was reinstalled, I dragged everything I had dragged to the external HD back into (what I thought) were their proper places on the MacBook Pro. Garage Band version 6.0.5 (428.5).
Then check please, if you still have your Loop Library and Instrument library.
All your GarageBand '11 loops should be installed in the system library - open the Library folder on your Macintosh HD:
The folder /Library/Audio/Apple Loops/ should contain all the loops you previously used. If you are seeing them there, but not in GarageBand, rebuild the Loop Index, see this document: Rebuilding the Loop Index in GarageBand
The second folder - /Library/Application Support/GarageBand/ - contains your instrument library. If instruments are missing as well, compare with your backup.