When he woke up, he was lying on a park bench.

Literal

Eyes [obj-を] awaken [discovery-と], he [topic-は] park [poss-の] bench [on-に] lying-was.

目を覚ます + narrative と reveals the scene discovered upon waking. 横になっていた ('was lying down') uses ~ていた for the state found. A classic story-opening pattern.