Dungeon Master Encyclopaedia

If you saved your game in a dangerous situation like being cornered by creatures that you cannot kill with your current power or if your saved game is broken (corrupted file or damaged disk that causes a SYSTEM ERROR 52 message when loading the game), don't panic.
You may not have to start a completely new game and lose your progress.
When you save your game, a backup copy of your previous saved game is always created. The backup file has a .bak extension while the last saved game has a .dat extension.
This backup copy allows you to restart from the previously saved location. You just have to rename or delete the .dat file. When loading the game will automatically try to load the .bak file if the .dat file is missing.