MY TOENAIL IS COMING OFF!!!!! HELP!!!!!?

ok so i wuz layin in bed last nite and i was putting a top coat on my toes so the nail polish wldnt chip. anyways i looked at my third toe on my left foot and my toenail has alomist completely fell off!!! it didnt hurt, and i nvr injured my toe! why is it coming off? how do i stop it? will it grow back? PLZ HELP!!!