。
She tried to laugh but couldn't.
Literal
She [topic-は] laugh-try-to did [but-が] no-good was.
~おうとした ('tried to') uses the volitional + とする in past tense for an attempted action. だめだった ('it was no good, it failed') is the colloquial way to report failure. The contrastive が connects effort with failed outcome. The situation suggests she wanted to appear cheerful but couldn't manage it.