。
She stopped singing.
Literal
She [topic-は] song [object-を] sing [nominalizer-の] [object-を] stopped.
~のをやめる is the standard 'stop V-ing' construction: V-plain + の (nominalizer) + を + やめる. The whole verbal phrase 歌を歌う ('sing a song') is wrapped in the nominalizer の, then taken as the object of やめる ('stop, quit'). Note the same を appears twice in the literal — once for the inner clause's object, once for the nominalized clause as the outer object. Japanese tolerates this structurally because the two are separated by clause boundaries. やめる has a wide reach: stop singing, quit a job, give up a habit.