Ten Years Later: The Man in The Iron Mask by Alexandre Dumas Chapter 56 Page 4

It often occurred that he forgot the day had half passed away, that the hours for the two first meals were gone by. Then he was awakened. He rose, descended to his shady walk, then came out a little into the sun, as though to partake of its warmth for a minute in memory of his absent child. And then the dismal monotonous walk recommenced, until, exhausted, he regained the chamber and his bed, his domicile by choice. For several days the comte did not speak a single word. He refused to receive the visits that were paid him, and during the night he was seen to relight his lamp and pass long hours in writing, or examining parchments.

Athos wrote one of these letters to Vannes, another to Fontainebleau; they remained without answers.

We know why: Aramis had quitted France, and D’Artagnan