"Sorry for my late reply. I was very sick yesterday"
=> 늦은 응답[회신] 미안해요. 어제 제가 많이 아팠어요. (Sorry for my late reply. ...)
=> 늦게 답해서 죄송해요. 어제 제가 많이 아팠거든요. (Sorry for replying late. ...)
* reply(n): 응답, 회신, 답장, 리플 (somewhat depends on the communication medium).
* reply(v): 답하다, 응답하다, 리플하다.
* (몸이) 아프다: be sick; 아팠다: was sick.