There could be a few reasons why your Fallout 3 game is not launching. One reason could be that your computer does not meet the minimum system requirements to play the game. Another reason could be that you do not have the correct version of Microsoft Windows installed. Finally, it is also possible that your game files may be corrupt.
If you are unsure of why your game is not launching, you can try contacting the game's support team for more help. You can also try searching online for solutions to common problems with the game.