Hypovolemic Shock due to Internal Bleeding: Given the patient's recent fall and subsequent development of hypotension and unresponsiveness, internal bleeding leading to hypovolemic shock is a highly plausible cause. The delay in onset of symptoms could be due to slow bleeding.

Sepsis: The patient's history of renal cell carcinoma and previous prostate cancer, along with CKD, makes him susceptible to infections, which could lead to sepsis, presenting with hypotension and altered mental status.
Dehydration and Electrolyte Imbalance: Patients with dementia may have poor oral intake, and those with CKD are at risk for electrolyte disturbances. These conditions could exacerbate after a fall, especially if the patient has been lying untreated for an extended period.
Adverse Reaction to Medication: Given the patient's multiple comorbidities, he is likely on several medications. An adverse reaction or overdose could lead to hypotension and unresponsiveness.

Intracranial Hemorrhage: Although less common than other causes, an intracranial hemorrhage after a fall in an elderly patient could present with delayed onset of symptoms, including hypotension and decreased level of consciousness. Missing this diagnosis could be fatal.
Pulmonary Embolism: The patient's history of cancer and recent immobility increase his risk for pulmonary embolism, which can present with sudden hypotension and decreased responsiveness.
Cardiac Causes (e.g., Myocardial Infarction): Given the patient's history of HTN and age, cardiac causes such as myocardial infarction should not be overlooked, as they can present atypically in the elderly.

Spinal Epidural Hematoma: Although rare, a spinal epidural hematoma could occur after a fall and present with hypotension and decreased level of consciousness due to spinal cord compression.
Adrenal Insufficiency: This could be a rare cause of hypotension, especially if the patient has been on long-term steroids for any condition, including his previous cancers or other inflammatory conditions.