。
She couldn't stifle her laughter.
Literal
She [topic-は] laughter [object-を] couldn't-endure.
こらえる (堪える) means 'to endure, to bear, to stifle' — stronger than 抑える, implying active effort to contain something. The potential negative こらえられなかった ('was unable to endure/stifle') tells us the laughter was too powerful to contain despite her best efforts.