。
A rockfall made the only road to the village impassable.
Literal
Rockfall [because-of-で] village [to-へ] lead [attributive-の] sole road [subject-が] pass-become-not.
通じる is a versatile verb meaning 'to lead to / connect with / be understood.' Here '通じる道' = 'the road that leads [to].' 通れなくなった = the compound 'become unable to pass': 通る ('pass') → potential 通れる → negative 通れない → + なる 'become' = 通れなくなる, past 通れなくなった. Complex verb stacking.