If you have not yet closed the replay (or killed minecraft instead of quitting it normally), you're lucky and have two options:
a) go to your .minecraft folder, inside the replay_recordings folder you will find your replay and a folder with the same name ending in ".tmp". copy your replay to another folder, then quit minecraft, then copy the file back
b) kill minecraft and the next time your start it, do not let it recover the replay
We'll add a confirmation dialog in the next release.