。
What would you do if you got hijacked?
Literal
By-chance hijack [with-に] encounter-[conditional-たら] how do-[question-か]?
~にあう (usually ~に遭う) is the 'encounter (typically unpleasantly)' construction — 事故にあう (get in an accident), 災害にあう (suffer a disaster), ハイジャックにあう (be hijacked). The に marks the unpleasant thing encountered. A distinctive Japanese pattern for 'being the victim of X.'