First, you need to find out why the crack happened. That will dictate how to fix it.

Sounds like your drywall was hung vertically to have a floor to ceiling crack that long. The cause is possibly a warped stud at that seam. Take a straightedge that is long enough to cover a few stud spaces, (at least 48"), and hold it horizontally centering it over the crack in the middle and see if it rocks. That will tell whether that stud is bowed out. Cracking can occur if the stud is extremely bowed in as well but typically more predominant when bowed out.