Here is the passage again:
Horrified, Sherri stared at her reflection in the mirror.
Hair sticking out in seventeen different directions,
a spot of whipped cream on her chin, and a green pen mark across her nose.
Why had her friends at the table not mentioned her disarray?
The second part is an afterthought fragment. It includes many good details but is missing a main clause, the most important component of a complete sentence.