the inability to recall the tune of a song you like. Opposite of the Last Song Syndrome (LSS) where a song can't get out of your head to the point that it becomes annoying, the Lost Song Syndrome (LoSS) makes you forget how the tune goes (but you still know the title, singer, lyrics, or even the choreography).
It makes the person consume a lot of time thinking, but the person will only remember the tune when he's tired of thinking about it anymore.
Guy1: What's your favorite song?
Guy2: 'Thunder' by Boys Like Girls
Guy1: Never heard of that before. Can you sing it?
Guy2: Sure.....mmm...uhmmm....*thinking*.....Oh shit, it's that lost song syndrome again!