Same thing happened to me.
An incredible Sakebi cover of ERROR by Vocaapple is down from youtube. ( No, it isn't on NicoNico. Guess the first place I would check.)
Luckily I downloaded it beforehand. My favorite teto cover of Lost one's weeping was removed, I sent a message to the author, not sure if I got a response. And the very best Teto cover of Discord was removed. I could only find it on some shady-ass site.