Probably a glitch, I've had this exact same thing happen. My most popular video has hundreds of comments, and I'd gotten a new one, so went to go check it, and to my utter horror I saw there were no comments. I thought "Oh the comments must be being filtered" in some way, but I checked again, and no comments whatsoever. The hundreds of comments from my video had all completely disappeared.
I checked one of my other videos, only to find the same thing. At this point though I started to panic a little less and thought "Well perhaps it's all of YouTube", so I checked another video outside of my channel, and unfortunately all their comments were still there and intact, so I began to think that my comments really were gone. I got as far as posting about it on the Google forums about YouTube, but by the time I was done with my post, the comments were back as if they'd never left.
So I suspect your situation is similar and just a glitch. Check back a little later and see if they're back