Cole Brown was a prominent character throughout the entire series, and she didn't "disappear" or leave. She remained a central figure in the show's dynamic, alongside Martin, Gina, and Pam.
There were some rumors and speculation about Tisha Campbell's departure in the final season, but ultimately, she was a core part of the show's cast until its end in 1997.
Perhaps you are thinking of another character from the show or perhaps there's a specific episode or storyline you're remembering? If you can tell me more about what you're thinking of, I might be able to help you further!