Hmmm... There is a slight chance that there may be a way to get your worlds back. If you are on windows, search %appdata%, click .minecraft, click saves, if your world is not in that folder it is probably lost for ever :/. If you are on mac, go to Macintosh HD, pres shift+command+g and type into the bar that appears ~/library then click ApplicationSupport. Find the minecraft folder. Go to saves. Same deal, if ur world is not in there it is gone. BUT, if you do find your world there, copy it onto your desktop. Force update your minecraft and then put the save back in the save file.
Good Luck :D
~miklvy