。
The act ended and the lights came on.
Literal
Act [subject-が] ended [and-連用形] lights [subject-が] turned-on.
幕 has a double meaning in theater: 'curtain' and 'act.' Here 幕が終わる clearly means 'the act ended.' 明かりがつく ('lights turn on') is the intransitive — lights turn on by themselves, contrasted with 明かりをつける ('to turn on the lights' — transitive).