。
There was money lying on the sidewalk.
Literal
Sidewalk [on-に] money [subject-が] was-fallen.
落ちている is the resulting-state ~ている on 落ちる ('fall') — 'is in a fallen state,' meaning 'is lying there.' Past: 落ちていた ('was lying'). This is the idiomatic way to describe things found on the ground: 手紙が落ちている (there's a letter on the ground), 財布が落ちていた (there was a wallet lying there).