It's lost forever, or at least until the end of the session. But players can share their cards, and the Invoker can also give out 3 cards. Finally, if a Complication Card is selected in a draw and is read as a particularly positive result, the Invoker can grant that card as an extra Action Card.
If a player loses all of their Action Cards and doesn't receive any more, they're out of the game - but that shouldn't happen unless they're taking serious risks, remembering that each player is only putting forward one Action Card at a time and that most checks don't involve risk of losing one.
I might rework this later, so let me know if you have any thoughts!