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.)