I'm sorry I missed the part where you say if someone is sad so to be literal and direct to your question. " why are you sad, wae seulpeoh ". Sorry for spelling I may be off.

u can say "무슨 일이 있어요" (what's matter with u/ what's up)

I do not know if my spelling will be good but waegeulae meaning what's up or museun il-iya meaning what's going on. I hope this helps you.