What should you do after blundering during a game? Let's say you are playing a game. Then suddenly, you make a bad move and e.g. blunder material. How should you react afterwards? What mind-set should you have? Can you even benefit from the blunder?
After a blunder, it helps to look objectively at the position once again and forget the history. You might have been better or worse; but that doesn't matter now after the blunder. What matters is how you go from here. 

There could be many possibilities of salvaging a draw or even snatching a win by making the position very complicated and causing your opponent to blunder (I sometimes jokingly call this a "counter-blunder"). The least you want to ensure is to not make it psychologically easy for your opponent to win after your blunder. Make it difficult for the opponent. Play good moves. Do not be gloomy lest you blunder again. 

Yes, there are times when you can even benefit from a blunder. A blunder may often give a player a kind of "license to thrill"! Now that you've blundered and your game is lost, you've got nothing more to lose! So why not embark on a nice sacrificial attack on the opponent's king (for example) and see if the opponent cracks under pressure? (A friend of mine actually tried this after blundering a piece and he won the game. Too bad I don't have the game).