YOU SAID:

I went to a shop today and when i came back my cat was gone

INTO JAPANESE

私は今日店に行き、私が戻ってきたとき私の猫は消えていた

BACK INTO ENGLISH

I went to the shop today and my cat was gone when I came back

INTO JAPANESE

私は今日店に行き、私が戻ってきたときに私の猫は消えていた

BACK INTO ENGLISH

I went to the shop today and my cat was gone when I came back