Hypovolemia: This is the most likely diagnosis given the sudden drop in blood pressure in a previously stable patient. Hypovolemia can be caused by dehydration, bleeding, or other fluid losses, which are common in elderly patients due to various factors such as decreased fluid intake, increased urine output, or gastrointestinal losses.

Medication side effect: The patient may be taking medications that can cause hypotension, such as antihypertensives, diuretics, or sedatives. A review of the patient's medication list is essential to identify potential culprits.
Infection: Elderly patients are prone to infections, which can cause hypotension due to sepsis or systemic inflammatory response syndrome (SIRS).
Cardiac causes: Cardiac conditions such as heart failure, coronary artery disease, or arrhythmias can lead to hypotension.

Myocardial infarction: Although less likely, myocardial infarction can present with hypotension, especially in elderly patients, and is a potentially life-threatening condition that requires prompt diagnosis and treatment.
Pulmonary embolism: This is another potentially life-threatening condition that can cause hypotension and requires immediate attention.
Adrenal insufficiency: This rare condition can cause hypotension and is often overlooked, but it is crucial to consider it in the differential diagnosis, especially if the patient has a history of steroid use or adrenal disease.

Neurogenic shock: This is a rare condition that can occur after a spinal cord injury or other neurological disorders, leading to hypotension due to loss of sympathetic tone.
Endocrine disorders: Other endocrine disorders, such as hypothyroidism or pheochromocytoma, can also cause hypotension, although they are less common.