Pulmonary Embolism with Lung Collapse: The combination of recurrent vomiting, altered sensorium, hyponatremia, hypokalemia, and the finding of lung collapse on NCCT chest could be indicative of a pulmonary embolism. The vomiting and altered sensorium could be due to systemic effects or secondary to hypoxia. Hyponatremia and hypokalemia could result from various factors including dehydration from vomiting and the body's stress response.

Intracranial Hypotension: This condition can cause vomiting, altered sensorium, and hyponatremia due to the depletion of cerebrospinal fluid (CSF) volume, potentially leading to increased ADH secretion. However, normal CSF analysis makes this less likely unless it's a case of spontaneous intracranial hypotension.
Adrenal Insufficiency: This condition can present with vomiting, altered sensorium, hyponatremia, and hypokalemia due to the lack of cortisol and aldosterone. The absence of fever does not rule out this diagnosis.
Pneumonia with Complications: Although there's no history of fever, pneumonia can sometimes present atypically, especially in older adults. The lung collapse could be a complication of pneumonia, leading to systemic symptoms.

Subarachnoid Hemorrhage: Despite the normal CSF analysis, a subarachnoid hemorrhage could potentially present with altered sensorium and vomiting. It's crucial to consider this diagnosis due to its high mortality and the need for urgent intervention.
Meningitis: Although the CSF analysis was normal, partially treated or early meningitis could still be a consideration, especially if the patient has been taking antibiotics.
Cerebral Vasculitis: This is a rare condition but could explain the altered sensorium and systemic symptoms. It would be critical to diagnose early due to its potential for severe complications.

Pituitary Apoplexy: A rare condition where a pituitary tumor undergoes hemorrhage or infarction, leading to acute hypopituitarism. This could explain the hyponatremia, altered sensorium, and potentially the vomiting.
Guillain-Barré Syndrome: An autoimmune disorder that could lead to altered sensorium if there's significant autonomic dysfunction. However, the primary symptoms would typically include ascending paralysis.
Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, presenting with altered sensorium and potentially vomiting. It's a diagnosis of exclusion but should be considered in the appropriate clinical context.