Close iTunes
Goto your iTunes folder
it's either under:
- My Documents\My Music\iTunes\
- \Users\username\Music\iTunes\
(Make sure that it's right one by looking at the Time & Date of the FIle (should be the same, as the last time you closed itunes)
Drag the iTunes Library file from the above location to the Desktop.
Open the Previous iTunes Libraries folder in the iTunes folder.
Locate the file named iTunes Library YYYY-MM-DD where YYYY-MM-DD is the date you upgraded iTunes (Year-Month-Day).
Drag this file to the iTunes folder (the enclosing folder).
Rename this file to iTunes Library.
Open iTunes.