。
A fallen tree had blocked the road.
Literal
Tree [subject-が] is-fallen-[and-て] road [subject-が] come-to-be-impassable-[was-ていた].
Two ~ている forms stacked via a te-form connective: 倒れていて ('be in the state of having fallen, and…') and 通れなくなっていた ('had come to no longer be passable'). The past resultative ていた means both states were in place at some past reference time. 通れない is the negative potential form of 通る ('pass through').