Cecile goes back to the convent. She's really young in the book, just a teenager (maybe mid-teens, not even late teens) and what happened spoiled her life.........or the life she could have had outside of the convent walls. But, IIRC, the book implies she's going to take the veil and spend the rest of her life as a nun.
I don't remember what happened to Danceny after the duel and he exposed Merteuil.
You didn't ask, but Madame de Tourvel dies and Merteuil ends up broke and contracts a bad case of smallpox which ruins her beauty.
You might prefer the end of the other movie version of the book, Valmont. In it Cecile doesn't lose Valmont's baby. Her marriage is still on. Cecile tells Valmont's older female relative (I forget in that version if it's his aunt or his grandmother) that she's carrying Valmont's child at his funeral.
I think that Merteuil flees before the terror starts and uses her diamonds as currency. I don't remember if it's before or after she loses an eye to smallpox.