Final answer:
Ludwig van Beethoven, transitioning from the Classical to the Romantic era in music, lost his hearing but continued to compose influential works.
Step-by-step explanation:
The composer who lost his hearing was Ludwig van Beethoven. Beethoven, a pivotal figure in the transition from the Classical to the Romantic era, began to lose his hearing in his late twenties and was almost completely deaf by the latter part of his life.
Despite his deafness, he continued to compose some of his most significant works during this time. His early compositions belong to the Classical period, with a clear classical style, while his later works are quintessentially Romantic, defining the shift in musical eras with his innovative compositions.