Surgical complication (e.g., direct nerve injury, inadequate decompression, or instability): This is the most likely diagnosis because paraplegia immediately following a lumbar laminectomy suggests a direct injury to the spinal cord or nerves during the procedure. The proximity of the surgery to the onset of symptoms strongly implicates a surgical complication.

Epidural hematoma: This is a known complication of spinal surgery that can compress the spinal cord and cause neurological deficits, including paraplegia. The timing of the onset of symptoms can vary, making it a plausible cause.
Infection (e.g., epidural abscess or meningitis): Post-surgical infections can lead to inflammation and compression of the spinal cord, resulting in paraplegia. While less common than surgical complications, infections are a significant concern in the post-operative period.
Spinal instability or hardware failure: If the surgical stabilization of the spine was inadequate or if there was a failure of the hardware used, it could lead to instability and compression of the spinal cord, causing paraplegia.

Spinal epidural hematoma: Although mentioned under other likely diagnoses, it's crucial enough to be highlighted here as well because missing this diagnosis can lead to permanent neurological damage if not promptly treated.
Anterior spinal artery syndrome: This is a rare condition but can occur due to injury to the anterior spinal artery during surgery, leading to infarction of the spinal cord and resulting in paraplegia. It's critical to consider this diagnosis due to its severe implications.
Venous infarction of the spinal cord: While rare, venous infarction can occur due to compression or injury to the venous drainage of the spinal cord, leading to ischemia and paraplegia.

Arteriovenous fistula or malformation: These vascular anomalies can be injured during surgery, leading to bleeding, compression, or ischemia of the spinal cord.
Spinal cord infarction due to atherosclerotic disease: Although rare, atherosclerotic disease affecting the arteries supplying the spinal cord could potentially lead to infarction and paraplegia, especially in patients with significant vascular disease.
Fibrocartilaginous embolism: A rare condition where material from an intervertebral disc embolizes and causes spinal cord infarction, leading to paraplegia. This is an extremely rare but possible cause of spinal cord injury.