A variety of issues may be caused by corrupted game files, e.g. immortal enemies, crashes, glitched graphics or inability to launch the game. If you have encountered such an issue, please verify the integrity of game cache and load a game save file created before this issue occurred (if possible).
GOG.com: 1. Launch GOG Galaxy and log in with your GOG.com account. 2. Go to the game's tab. 3. Click MORE > Manage installation > Verify / Repair > VERIFY GAME.
Steam: 1. From the Library section, right-click on the game and select Properties from the menu. 2. Select the "Local files" tab and click the "Verify integrity of game cache..." button. 3. Steam will verify the game's files - this process may take several minutes.
Origin: 1. Select the My Games in Origin. 2. Right-click on the game. Select "Repair Game."