Hmm... it appears that when you delete a world... it doesn't show up in the Recycle bin...
Just let this be a lesson for the future: Always do backups !
Just be glad you didn't lose something really important... like a five page essay for college!
Oh and btw, if you're uncertain about doing backups for Minecraft worlds... do this (on Windows):
Start -> Run - %appdata%
goto the .minecraft folder
goto the /saves/ folder
copy the world that you want to backup.
paste it to a backup location.
Hope this helps!