Save file got Deleted when I backed up

 When Minecraft updated to 1.6.4 my save file asked if I wanted to back it up I press the backup and load button and then continued to play Minecraft for several hours. Today when I try to play that file again it wasn't there so I figured it was in the backups when I opied the file back to saves and loaded up the file It was the save before I start to play for several hours and I've looked in my recycling bin and it doesn't seem to have been deleted I tried this with another world in a didn't seem to have a problem but it was an older save file so I don't know if that could have changed anything And I wasn't playing for several hours