Nakkivene

There's no "world.dat" file in the world folder...

Hello, I have been playing Hexxit with my friend for a few weeks now. We just built our base and we had lots of good stuff, but then I messed up badly. When I was about to start the server, I opened the "minecraft_server.jar" file. Now almost everything we had is gone, except for the Vanilla items. Is there any way to undo this? If there is, please let me know, thank you! Regards