The writer forgot all about M. Hamel's ruler because he was deeply moved by the emotional farewell speech of his teacher, M. Hamel. Ashamed of his own lack of preparation and realizing the importance of learning, the writer was overwhelmed by the teacher's sincerity and devotion.
Final Answer:
\[
\boxed{\text{The writer was deeply moved by M. Hamel's farewell speech and forgot about the ruler in his hand.}}
\]