My explanation:
Barnabas, fed up with Willie spending too much time on Roxanne and not enough on helping him, kidnapped Roxanne and chased her around Collinwood. She escaped via the Stairway Through Time into... oh, let's say 1954. She married a man by the last name of Drew and had a daughter, also named Roxanne. I'm positive the writers had all this in mind.
All kidding aside, I think the writers just forgot about her. They completely forgot about Angelique pulling a Josette and Jeremiah on Maggie and Quentin, too, and that was around the same time, no?