Q: I lost all my music on iTunes, and all it says is this: The song “(music title)” could not be used because the original file could not be found. Would you like to locate it?

Hey xxCreepyFiaxx,

First, I would try the following suggested steps for when you don't see content in iTunes after updating:

1. Quit iTunes.
2. Download and install the latest version of iTunes.
3. Use the Finder (Mac) or Windows Explorer (Windows) to go to the iTunes folder that contains the iTunes library files:
   
   Mac OS X
   /Users/username/Music/iTunes/
   
   Microsoft Windows Windows XP and Windows 2000
   \Documents and Settings\username\My Documents\My Music\iTunes\
   
   Microsoft Windows Windows Vista and Windows 7
   \Users\username\Music\iTunes\
4. Drag the iTunes Library file from the above location to the Desktop.
5. Open the Previous iTunes Libraries folder in the iTunes folder.
6. Locate the file named iTunes Library YYYY-MM-DD where YYYY-MM-DD is the date you upgraded iTunes (Year-Month-Day).
7. Drag this file to the iTunes folder (the enclosing folder).
8. Rename this file to iTunes Library.
9. Open iTunes.

You should now see your missing content in iTunes.

via: No content shows up in iTunes after updating
http://support.apple.com/kb/TS1967

If that doesn't resolve the issue, then I'd go through the steps in here:

iTunes: Finding lost media and downloads

http://support.apple.com/kb/TS1408

Have a good one,

Delgadoh