Here is the passage again:
Struggling up three flights of stairs, her arms
filled with the heavy texts for four classes, her backpack loaded with notebooks
and other supplies. Jamala huffed and puffed. Never again would she
register for all of her classes back-to-back on the same days of the week.
The first part is a participle phrase fragment. Among all of the many words, there is no main clause, the essential item in a complete sentence.