。
The curtain caught fire.
Literal
Curtain [target-に] fire [subject-が] lit.
Similar to 火がつく but with 点く (つく), a reading-variant kanji more specifically used for lights and fires igniting. 火が点く = 'fire lights (attaches).' Compare with the transitive 火をつける ('to set fire to'). The に marks the location where the fire started, not the target of an action.