English proverb for when a solution comes too late

You were close with the cow and the barn door. It should be

It's too late to shut the stable door after the horse has bolted.

Another is

It's easy to be wise after the event.

Taking a saying from a language you do not speak and trying to "translate" it is hard enough, given the challenging nature of idiomatic words and expressions. Coming up with an equivalent saying in a language you do speak often compounds the difficulty.

Frankly, other contributors to this post have beaten me to the punch with "20/20 hindsight," "Too little, too late," and "A day late and a dollar short."

While this may not be an answer to your question, it is somewhat apropos nevertheless. The French, I am told, have a saying that could be translated into English, roughly, as "brilliance on the staircase"; that is, thinking of the right thing to say when it's too late to do any good. Say some smart aleck at a party (or wherever) aims a snide comment at you. You are temporarily rendered speechless or you blurt out something feckless like "Oh yeah?!" Then on the steps as you leave the party you think of the perfect riposte, but it's too late. (D'oh! as you smack your forehead). Is there an English equivalent?

Well, how about "That ship has sailed"? Or, "Monday morning quarterback?" Or, "We get too soon old, and too late smart"? Or, "Why didn't I think of that!?"? Or, "You can't reinvent the wheel"? Or, "You can't un-fire a gun"? Or, "You can't un-ring a bell"? Or, "That's water over the dam"? Or, "Beating a dead horse"? Each one of the foregoing has a flaw or two, I suppose. Any other suggestions out there?

"A day late and a dollar short" is an expression used in situations you describe. Essentially, the solution is available but is of no use because of the delay in formulating it/

I love the reference to "brilliance on the staircase". Freud called such brilliant responses "Stair step thoughts"

Most others here refer to being out of luck but not an actual solution in hand. One I heard was "A perfect landing at the wrong airport."

I've heard people simply say, "The horse is out of the gate." Makes for a shorter version of coleopterist's answer. May be a southern U.S. colloquialism though.

Borrowed from French, the expression esprit de l’escalier, or esprit d’escalier, literally wit of (the) staircase, denotes a retort or remark that occurs to a person after the opportunity to make it has passed.

(from wordhistories.net)