It is not exactly known when Beethoven became completely deaf, but it is thought that his Ninth Symphony is one of the works completed while he was deaf.
To hear such emotion and mastery of sound from one who could not actually hear it outside of his own mind is a miracle and a curse. Beethoven had such mastery of notes by the time he was deaf he know exactly what it would sound like in his own mind and was able to put the notes down for the orchestra to read!! Amazing!!