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She's worried she might have failed her end-of-year exam.
Literal
She [topic-は] year-end exam [target-に] failed [I wonder-のではないか] [quotative-と] is-worrying.
The construction ~のではないか ('isn't it that...?' / 'I wonder if...') is a soft suspicion-marker — literally 'is it not the case that X?' but functionally 'I worry / suspect that X.' The pieces are の (nominalizer) + で (copula te-form) + は + ない + か (question particle). Layered with と心配している ('is worrying that'), it expresses anxious uncertainty about a specific outcome. 学年末試験 ('year-end exam') is a Japanese academic year landmark — the school year runs April to March, with major exams clustering in February-March. 試験に落ちる ('fail an exam') uses 落ちる ('fall'), the natural metaphor: failure as falling short of the bar.