After the storm, the road was blocked by fallen trees.

Literal

Storm [possessive-の] after, road [topic-は] fallen trees [with-で] was-blocked.

倒れた木 (fallen trees) uses the past tense of 倒れる as a modifier. 塞がれた is the passive of 塞ぐ (to block). で marks the instrument/cause of the blocking.