。
Our house was knocked down by the storm.
Literal
Violent-wind [by-で] our house [subject-が] was-toppled.
で marks the storm as the cause/instrument of the passive action. 倒された is the passive of 倒す (to knock down/topple). The が marks 家 as the grammatical subject of the passive verb — in Japanese passives, the thing affected takes が while the agent/cause takes で or に.