The tone in Petrarch's Sonnet 292 undergoes several shifts. The first is between Stanzas 1 and 2 where the poet switches from awed reminiscence of the special distinction his love for the woman spoken of gave him among other men to a joyful reminiscence while thinking of her golden angelic qualities and the paradise she made of Earth.
The most dramatic and important tonal change is at Line 8 where he discloses that this angelic woman is dead. The final three tonal changes are in the last two Stanzas. The first change is to a tone of lament that he still lives and has cherished light while she has none. The tonal change is to a strange sudden breathlessness, as though his poetic lungs simply run out of air. He describes it as vein of poetic art running dry in the same way that a vein of gold in a mountain might suddenly turn out to be empty. The final change in tone comes when he say his lyre (the musical instrument by which poets sing their poems) has dissolved into tears. Which leads to the final tonal change as the poet dissolves in tears as well.
I believe that this is a sonnet of mourning -- one in which the speaker's love has died. Because his love has died, he has also lost his will to practice his art -- to write poetry.
Since this is a Petrarchian sonnet, the first 8 lines are distinct from the last 6. In this sonnet, the first 8 describe the speaker's love and talk about how she is now dead. The last six lines talk about the effects of her death. The speaker no longer even wants to write poetry and like his life is more or less over.