Idiopathic Sudden Sensorineural Hearing Loss (ISSNHL): This condition is characterized by a sudden loss of hearing, often in one ear, and can be temporary or permanent. The fact that hearing returned in one ear but not the other is consistent with this diagnosis. The dull appearance of the eardrum could be related to middle ear effusion or other factors not directly causing the hearing loss.

Middle Ear Effusion: The dull appearance of the eardrum bilaterally could indicate the presence of fluid in the middle ear, which can cause hearing loss. The resolution of hearing in one ear could be due to the spontaneous resolution of the effusion.
Acoustic Neuroma or Other Tumors: Although less common, tumors affecting the auditory nerve can cause sudden hearing loss, especially if they are growing and compressing the nerve. The fact that the hearing loss is unilateral (now) could suggest a localized problem such as a tumor.
Viral Labyrinthitis: This is an inner ear disorder characterized by inflammation of the part of the inner ear responsible for balance and hearing. It could cause sudden hearing loss and might be more likely if there were other symptoms such as vertigo, but these are not mentioned.

Stroke or Cerebrovascular Accident (CVA): While less common as a cause of isolated hearing loss, a stroke affecting the auditory pathways in the brain could present with sudden hearing loss. Given the potential severity and the need for urgent treatment, this diagnosis must be considered.
Multiple Sclerosis: This autoimmune disease can affect various parts of the central nervous system, including the auditory pathways, leading to hearing loss. Early diagnosis and treatment can significantly impact the disease's progression.
Meningitis: Inflammation of the protective membranes covering the brain and spinal cord, which can be life-threatening and requires immediate medical attention. Hearing loss could be a symptom, especially if the infection involves the inner ear or auditory nerve.

Autoimmune Inner Ear Disease: A rare condition where the body's immune system attacks the inner ear, leading to hearing loss. It can be bilateral but often starts in one ear.
Cogan Syndrome: A rare disorder characterized by recurrent corneal inflammation and vestibular-auditory symptoms, including hearing loss. It's an autoimmune condition that can be challenging to diagnose.
Sjögren's Syndrome: An autoimmune disorder that can affect various parts of the body, including the ears, leading to hearing loss among other symptoms. It's more commonly associated with dry eyes and mouth but can have a wide range of manifestations.