Ok I have had this happen a bunch of times. Mostly with mods. Ok number 1 If you have the jar file still in the trash recover it. then just hit you're login. If thats not in the trash and is gone completely then I have a few steps. Ok number 1 for this problem Close minecraft completely. If it still says close the file and try again then Its probably gone for good.Then delete the whole .minecraft ( save your texture packs and saves if you want).
Last hit minecraft and press options and force update.
