You could allow your GC to explain in his/her letter.
You could explain by submitting a supplemental statement.
I would not recommend using your personal essay to delve into this. The point of the personal essay is to let them find out more about you, about what sets you apart from other applicants. Using that precious opportunity to draw attention to, and explain, a semester of bad grades would not be advisable imo.
For good advice on essays, see
U.Va. Office of Admission Essays