YOU SAID:

my face fell off when I sneezed

INTO JAPANESE

私の顔は落ちたときくしゃみ

BACK INTO ENGLISH

My face fell when sneezing

INTO JAPANESE

私の顔に落ちたときにくしゃみ

BACK INTO ENGLISH

Fell on my face when sneezing

INTO JAPANESE

くしゃみをする場合、私の顔に落ちた

BACK INTO ENGLISH

If you sneeze and fell on my face

INTO JAPANESE

くしゃみをして私の顔に落ちた場合

BACK INTO ENGLISH

If you sneeze and fell on my face