Currently Being ModeratedJan 18, 2013 1:45 AM (in response to Hazel1)
Itunes mearly reads a folder where all your songs, movies and such are. Once you add music from a folder on your PC to itunes that folder cannot be altered in anyway or itunes cannot find the songs. Do you only buy from itunes, rip from cd, or pirate? sounds like your stuff got deleted off your computer. If option in unclicked the music default location is C:\Users\YOUR NAME\Music\iTunes\iTunes Media\Music.
Currently Being ModeratedJan 18, 2013 2:45 AM (in response to Hazel1)
Empty/corrupt library after upgrade/crash
Hopefully it's not been too long since you last upgraded iTunes, in fact if you get an empty/incomplete library immediately after upgrading then with the following steps you shouldn't lose a thing or need to do any further housekeeping. In the Previous iTunes Libraries folder should be a number of dated iTunes Library files. Take the most recent of these and copy it into the iTunes folder. Rename iTunes Library.itl as iTunes Library (Corrupt).itl and then rename the restored file as iTunes Library.itl. Start iTunes. Should all be good, bar any recent additions to or deletions from your library.
See iTunes Folder Watch for a tool to catch up with any changes since the backup file was created.
When you get it all working make a backup!
Should you be in the unfortunate position where you are no longer able to access your original library, or a backup of it, then see Recover your iTunes library from your iPod or iOS device.