Should I apologize to my professor for not attending a lecture? Long story short, I was unable to attend a lecture of a course I'm interested in. I met my professor directly at her office and asked questions and extra material for her course. Moreover since I'm interested in her course, I've read some extra notes, books and papers. After stating an advanced question she suggested me to read some extra papers and extended bibliography.

How can I "apologize" my professor for not being able to attend the lecture, after showing such excitement? Should I send her an e-mail or talk to her directly? (I guess face-to-face discussion is preferred.)
When you see her next, say something like


  I apologize for not showing up for lecture last week. I had a 
  doctor's appointment/seminar talk/committee meeting/friend's PhD 
  defense I had to attend.


Nothing more is needed than just to convey the message that you had a good reason to not attend class. Professors are busy people, they know that missing meetings is a fact of life. It only becomes awkward if you stayed away from class because you wanted to watch the last episode of Walking Dead/went out drinking with your friends/etc.