I'm usually pretty good at remembering what my characters look like, etc. Now and then I change an eye color or two, but my internal picture of who these people are is pretty much set before I start writing. Not any more.
Once I lose that picture, I find it hard to get the characters back. Perhaps because I visualize each scene, (art history major here), once the setting and the people in it are confused, the story may as well jump off a cliff. Several early works have suffered that awful fate. Recently, I picked up a WIP that needs revisions and had been resting in the corner of my mind, and I found I couldn't see the story anymore. The people had taken a hike. I don't know if it's a result of an overload in my circuits (too much going on here to mention) or that they just decided to teach me a lesson.
If so, I have learned it. From now on, I'm going to make detailed notes about the characters and every little characteristic. (Sorry if there's an overload of "character" in this post. Can't be helped.) See if they can get away from me now!