A dead hand can't win anymore, full stop, usually because a tile it needed already got discarded and passed by, an opponent's exposure blocked it, or it just stopped matching anything on the current card. Once that happens, you're still in the game technically, still discarding, but you're out of the running for that round.
It happens more than most beginners expect, almost always from chasing a hand a few turns past the point where it was realistically still alive. There's no trick to fixing it, really. Just keep half an eye on what's been discarded, and be willing to admit a hand's dead a little sooner than feels natural.
A dead hand isn't some big failure. It happens to strong players constantly. Catching it early is really just about not spending three more rounds chasing something that was never going to happen.
Keep going with what an exposure is or our step-by-step guide to American mahjong.