Final answer:
Toward the beginning of Beethoven's middle period, one major event was his struggle with the onset of hearing loss, leading to the Heiligenstadt Testament in 1802 and marking a transition in his musical style towards the Romantic era.
Step-by-step explanation:
One significant event that took place toward the beginning of Beethoven's middle period was the composer's acceptance and adaptation to his worsening hearing loss. Around 1801, Beethoven began to notice his hearing was declining. By 1802, he wrote the famous Heiligenstadt Testament, a letter in which he expressed his anguish over his growing deafness.
This personal crisis deeply impacted his life and work, leading to the creation of more introspective and emotionally powerful compositions. This period also marks the time when Beethoven transitioned from the Classical traditions into what would become the Romantic era of music, characterized by individual expression and emotional depth in works such as his Symphony No. 3, titled 'Eroica', which was completed in 1804.