It is clear from Chapter Six of Book the First that the long years of captivity have had an incredibly negative impact on the life of Dr. Manette, as this chapter shows that he has lost all sense of his former identity, and indeed of having any identity at all apart from being known as "One Hundred and Five, North Tower," and mending shoes. Note the first description we are given of him as Monsieur Defarge leads Mr. Lorry and Lucie Manette into his room:
The faintness of the voice was pitiable and dreadful. It was not the faintness of physical weakness, though confinement and hard fare no doubt had their part in in it. Its deplorable peculiarity was, that it was the faintness of solitude and disuse. It was like the last feeble echo of a sound made long and long ago. So entirely had it lost the life and resonance of the human voice, that it affected the senses like a once beautiful colour faded away into a poor weak stain. So sunken and suppressed it was, that it was like a voice underground. So expressive it was, of a hopeless and lost creature, that a famished traveller, wearied out by lonely wandering in a wilderness, would have remembered home and friends in such a tone before lying down to die.
Note how this barrage of description presents Dr. Manette as being literally a shadow of his former self. He has not used his voice much at all, and his voice is compared to being a once vibrant colour that is now nothing more than a pale "weak stain." His voice is reminiscent of death and abandonment. The way in which he has no memory of being anything else than a prisoner and a mender of shoes clearly shows the way that his captivity has impacted him mentally as well as physically, reducing him to nothing more than a pair of "haggard eyes" and a weak, frail, old man. It is only the love and care of his daughter that manages to resurrect him, bringing him metaphorically back to life, as Mr. Lorry's cryptic message explains.