We actually cannot know with certainty whether or not Scrooge really encountered the four ghosts (including the ghost of Jacob Marley, his former business partner) or whether he only dreamed that he did. He certainly believes that he really did travel from his bedroom into his past, into the streets of London, and into the future.
Ultimately, does it really matter whether or not Scrooge actually met the ghosts? I don't think so. Because he believes that he did, he's been inspired to change the way he lives his life, to treat his employee, Bob Cratchit, more nicely, to foster a relationship with the only family he has left, and to respond to the needs of the poverty-stricken with humanity. Maybe Scrooge only dreamed that the ghosts visited him, but the point is that he has learned that the most important part of being alive is helping and enjoying other people, and this lesson is valuable regardless.