There is only (i think) two options to get back your world.
1. Metioned by Roman "You can restore a backup on the backups page."
&
2. By uploading you world to server.
If it's not a first option
then maybe you have downloaded your previous loads of world.
Im always downloading a world after i did something big to my world. Accidents can happend every time.
If non of those things is possible im sad to say it but there is no other option as far as i know.
You can set "autosave" option to "on" so next time your save will be safe.
Good Luck ! ~Jacob
Jacob.