There is a feature of electronic futures markets called a "limit-down" that dictates that certain indices are not permitted to fall beyond a certain point in overnight trading. This is sort of the financial equivalent of a circuit breaker tripping to avoid a fire.
The S&P 500 got down-limited last night.
Here's some advice - change your 401(k) password to something random, then forget it. You shouldn't be looking at it more often than once a quarter. Seriously.