The trip was canceled for two reasons. Firstly, the weather was terrible, and secondly, several key people got sick.

secondly 是副词，由序数词 second（第二）加上后缀 -ly 构成，用于列举时表示"第二点"或"其次"。在列举事项时，常用 firstly...secondly... 这样的结构来依次说明原因或要点。