Viral Rhinitis (Common Cold): This is the most likely diagnosis given the symptoms of a severe cold and loss of smell, especially when a Covid test is negative. The common cold is a viral infection that can cause inflammation in the nasal passages, leading to a loss of smell.

Influenza: Similar to Covid, influenza can cause severe cold-like symptoms, including loss of smell. Although less common, it's a plausible diagnosis, especially during flu season.
Sinusitis: This condition, which involves inflammation of the sinuses, can cause a loss of smell along with other cold-like symptoms. It can be acute or chronic and may require further investigation.
Allergic Rhinitis: Allergies can cause nasal congestion and inflammation, potentially leading to a loss of smell. This diagnosis is more likely if the patient has a history of allergies.

Meningitis: Although rare, meningitis is a potentially life-threatening condition that can present with severe headache, fever, and loss of smell. It requires immediate medical attention.
Tumor or Mass in the Nasal Cavity or Brain: A tumor in the nasal cavity or a mass in the brain, such as a meningioma, can cause a loss of smell. These conditions are rare but critical to diagnose early.

Parkinson's Disease: Some patients with Parkinson's disease may experience a loss of smell early in the disease process. However, this would typically be accompanied by other neurological symptoms.
Multiple Sclerosis: This autoimmune disease can affect various parts of the brain, including those responsible for smell, leading to a loss of smell in some cases.
Sjögren's Syndrome: An autoimmune disorder that can cause dryness in the eyes, mouth, and nose, potentially leading to a loss of smell.
Head Trauma: Significant head trauma can damage the olfactory nerves, leading to a loss of smell. This would typically be associated with a history of recent head injury.