A clarification, while we have editors who edit with other animation footage (video games, Ponies, Avatar), the purpose of this community is to catalogue and discuss ANIME music videos, as in Japanese animation made (originally) for Japanese domestic audiences. If the animation source of this particular video was not Japanese animation, or not animation meant for a Japanese audience (even if it was animated at a Japanese firm/studio), then we may not have even heard of it, let alone know it from the limited description you can remember.
If it is an actual AMV, well, AMV requests are normal, however, unless the video is very old (2002 or older) or very new *and* very famous, the likelihood of finding it if it is not made by an Org member and catalogued at the Org is very, very small. The community on the Org and even in the entirety of the North American con scene hasn't been small enough to cover all AMVs in a number of years, and that's not addressing the hobby as a whole. There are still ftps, fservs, livejournals (AbsoluteDestiny's comes to mind), and a number of country specific international sites. And many YouTube videos, probably tens of thousands or hundreds of thousands at this point, are never known to the members of the Org at all. It's just not possible.
Then there's people like me. I rarely use YouTube at all, even to watch videos. If someone directly links me, that's one thing. If I am searching for some sort of technology review or repair demonstration, perhaps. Compared to most people I know, however, my YouTube use is very limited. I'm hardly the only one in this position.
This is all a very complicated and detailed way of saying that we probably can't help you. Not that we won't, not that we don't want to, but that we actually can not. It isn't possible.