I updated iTunes and now I've lost most of my songs! Can I get them back?

Download your past purchases - Apple Support

Recover your iTunes library from your iPod or iOS device.

See turingtest2's user tip on Empty/corrupt iTunes library after upgrade/crash - this should enable you to restore your library to its pre-update state. The links in ManSinha's post will help if restoring your library database from the Previous iTunes Libraries folder isn't effective.