There have been rare reports of instructors who are in non-playable areas of the map (like inside a wall, etc.) or invisible.

Search around a bit for the instructor (look on your mini-map for green dots, or use the "Tracking" skill if you have it). If you locate the instructor, even if it is in a non-playable area, you may be able to talk to him and he may pop back into the map.

Your other alternative is to load your last saved game before entering that level. But look around for him first, sometimes they can wander around a bit.