You logged in. You should not have logged in. But you did, and there it was.
A number smaller than the last number.
And here is the part that makes it sting: you did not do anything. You did not sell. You did not gamble. You went to work, you contributed the same amount you always contribute, and the account thanked you by shrinking.
That feels like a betrayal. It is not. But "it is not a betrayal" is a terrible answer on its own, so let's do better than that.
There are exactly six reasons a 401(k) loses money. Five of them are normal and one of them is quietly robbing you. The whole trick is telling them apart.
By the end of this you will be able to look at your own statement and say, with some confidence, "that one is the market, that one is me, and that one needs to stop."