I saved a game in an unwinnable spot by accident. I have played this game for several years and my backup save is not winnable either. Is it possible to retrieve an earlier save game?

If this is a console system's memory card, the answer is no.

If this is a PC game, then the answer is maybe. You may be able to recover an earlier version of the savegame file using a utility like Restoration (http://www.snapfiles.com/get/restoration.html).

Most likely however, your save game is gone. Sorry dude.

The only other option (if it was a PC game) is to look for something like a saved-game hacking program. The saved game is nothing but a hexadecimal file storing all information about your game. Some people have written programs for some save-game files that allow you to tweak character stats, inventories, and even locations and locked doors.

There are a couple of games out there that were buggy when released, and required the use of saved-game hack programs to actually complete. A little Google searching might tell you if it's possible in your case.