Who is asking for a backup file exactly?
Losing the savegame is of course not supposed to happen oO Unless you press the "delete" button in the singleplayer menu, there is no way the game deletes a savegame (so this might only be caused by an external influence - file loss, harddrive issues, malware).
The game saves the world all 10 seconds (and also when you leave the game), if an error occurs while saving, you will see a message in chat. What antivirus program do you use exactly? Some programs (like Comodo) run the game in a "sandbox", i.e. the game is not able to write on the harddrive.
You can make sure if there is still the world file in place, just go into the game directory (steamversion: rightclick on rw in steam -> properties -> local files -> browse local files) into the "Worlds" subfolder, there should be a folder with the name of your world. This folder is supposed to contain a ".db" file, as well as another subfolder "worldParts" (which contains more .db files).
Can you still select your world in the singleplayer menu?