Hypopituitarism secondary to meningioma resection: This is the most likely diagnosis given the patient's history of meningioma resection. The surgery could have damaged the pituitary gland, leading to decreased ACTH production. The normal urine cortisol level suggests that there is still some adrenal function, but the low ACTH indicates a problem with the pituitary-adrenal axis.

Adrenal insufficiency secondary to exogenous steroid use: Although the patient had a meningioma resection, it's possible that she was treated with exogenous steroids post-operatively, which could have suppressed her adrenal function. The normal urine cortisol level might not accurately reflect her adrenal function if she has been on steroids.
Pituitary apoplexy or other pituitary lesions: Other pituitary lesions or apoplexy could also lead to hypopituitarism and low ACTH levels. Given the patient's history of a brain tumor, it's possible that there are other lesions affecting the pituitary gland.

Cushing's syndrome in remission: Although it might seem counterintuitive, Cushing's syndrome in remission could present with low ACTH and normal urine cortisol. This is because the adrenal glands might still be recovering from the excess cortisol production.
Adrenal crisis: An adrenal crisis is a life-threatening condition that requires immediate attention. Although the patient's urine cortisol is normal, an adrenal crisis could still occur if she is not producing enough cortisol in response to stress.

Congenital adrenal insufficiency: This is a rare condition that could present with low ACTH and normal urine cortisol. However, it would be unusual for this condition to present for the first time in a 41-year-old woman.
Familial glucocorticoid deficiency: This is another rare condition that affects the production of cortisol. It could present with low ACTH and normal urine cortisol, but it would be an unusual diagnosis in an adult patient.