Check your folder where you usually keep media (usually in your iTunes folder). Are the media gone too, or just the library entries?
Is there anything else we should know about your iTunes use such as you subscribing to Match?
Now is a good time to remember if you have an up to date backup you don't have to worry about this kind of thing.
Two slightly different approaches as long as the media are still around:
Restoring old library file after doing an iTunes update - https://discussions.apple.com/message/17789607
Empty/corrupt library after upgrade/crash - https://discussions.apple.com/message/19812487 - turingtest2 post on restoring library archived at time of last iTunes application update.