What dream has Mr. Lorry awoken from in A Tale of Two Cities?

I assume from your question that you're referring to Mr. Lorry's dream in chapter 3, "The Night Shadows." As he rides in the mail-coach, Mr. Lorry "nodded in his place, with half-shut eyes." As Dickens describes,

Though the bank was almost always with him, and though the coach (in a confused way, like the presence of pain under an opiate) was always with him, there was another current of impression that never ceased to run, all through the night. He was on his way to dig some one out of a grave.

This "someone" never becomes clear to Mr. Lorry, though the faces in his vision are "the faces of a man of five-and-forty years" and look ghastly in their "worn and wasted state." Mr. Lorry repeatedly asks this person how long he has been buried for, to which the "spectre" replies, "Almost eighteen years." When Mr. Lorry tells the spectre that he has been "recalled to life" and asks, "I hope you care to live?" the ghost responds, "I can't say."
Next in the dream, Mr. Lorry asks, "Shall I show her to you? Will you come and see her?" and it is here that the dream varies: sometimes the spectre replies, "Wait! It would kill me if I saw her too soon"; other times he says "Take me to her"; or else he is confused, saying, "‘I don’t know her. I don’t understand."
After this conversation occurs, the Mr. Lorry in the dream digs the spectre out of his grave, at which point "he would suddenly fan away to dust" and Mr. Lorry would "start to himself, and lower the window, to get the reality of mist and rain on his cheek." The exchange about the spectre having been buried for eighteen years recurs several times, as well as the spectre's seeming ambivalence about coming back to life.
When Mr. Lorry finally truly awakes, it is dawn, and the "shadows of the night were gone." As the chapter closes, Mr. Lorry marvels at the horror of being buried alive for eighteen years.