Currently Being ModeratedJan 27, 2013 3:14 PM (in response to hilki)
How did you reinstall? Did you migrate from a TimeMachine backup?
That does not transfer your Instrument Library, see Christoph Drösser's post:
To fix this, replace the complete Instruments Library by the corresponding folder from your backup disk:
/Library/Application Support/GarageBand/Instrument Library/
If you are trying to reinstall GarageBand again from the AppStore and cannot download the additional loops and instruments, then make sure, you sign into the AppStore with your AppleID before you try to initialize the download from GarageBand. When prompted in GarageBand to download, opt for "download later". Wait a bit, then quit the AppStore, and open it again, to reload the Purchases page and look for an update.
But that alone may not suffice.
To make the additional content appear you may have to do the full monty and deinstall GarageBand completely, reboot, then reinstall.
See Dagon's post here:
LéonieOS X Mountain Lion, 2.4 GHz I.Core i7, 16GB, 500GB SSD