I haven't had the experience of looking into the mirror and not being able to see the reflected image but many many many times I have had the experience of not being able to maintain looking at the reflection because it was too disturbing to see the body reflected there instead of the internal image of self. So I can see.... but I can't look. I'm not sure if that is what you experienced since you said you couldn't see.
I definitely attribute my inability to look (when it happens, it isn't all the time) to my dissociation.