This is the 10460th most frequent Polish word.
czyją
"Czyją" means "whose" (feminine singular or plural, accusative case).
In this context, "czyją" asks about the person responsible, modifying the noun "winą" in the instrumental case.
Nie wiem, czyją to było winą.
I don't know whose fault it was.
The word "czyją" is used here as an interrogative pronoun asking about possession and modifying the noun "książkę" in the accusative case.
Czyją książkę zostawiłeś na stole?
Whose book did you leave on the table?
The word "czyją" here identifies the owner of the decision in question, modifying the noun "decyzją" in the accusative case.