1. Please try the following first:

    1. Right click Bastion in Steam Library -> Properties
    2. Select the 'Betas' tab
    3. Select 'legacy' branch

If that doesn't help, please try the following:
 
  1. Please open the Task Manager by pressing Ctrl + Alt + Delete and terminate any instances of the game before trying to start it again. Alternatively, simply restart your machine.

  2. Next, try closing any other programs that you have open as it's possible one of them is conflicting with the game.

  3. It's possible that some of your game files were corrupted during the download. Please try "verifying the integrity of your game cache", which will try to detect file corruption and redownload them as needed. Instructions are here:

    https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335

  4. Next, please try navigating to the game's install directory and run the following files:

    dotNetFx35setup.exe
    dxwebsetup.exe
    xnafx31_redist.msi

  5. Then try running Bastion.exe directly, in Windows XP compatibility mode, and with administrative privileges.