If you lost your playlists with an iTunes upgrade, they are backed up in your Previous iTunes Libraries folder.
1. Close iTunes
2. Send your old library file to the Recycle Bin (the default file is in My Music\iTunes\iTunes Library.itl)
3. Navigate to the My Music\iTunes\Previous iTunes Libraries folder
4. Do a directory listing and find the library file with the newest date ex: iTunes Library <date>.itl
5. Copy that file up one level to My Music\iTunes (or your custom location)
6. Rename the file to remove the date from the file name ex: iTunes Library.itl
7. Open iTunes