Thanks for the question. I understand that your iTunes library does not include music that is on your computer. The following article may help to resolve your issue:
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
Microsoft Windows Windows XP and Windows 2000
\Documents and Settings\username\My Documents\My Music\iTunes\
Microsoft Windows Windows Vista and Windows 7
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.