Bell's palsy occurs when the nerve that controls the facial muscles is swollen, inflamed, or compressed, resulting in facial weakness or paralysis. Exactly what causes this damage, however, is unknown.
Most scientists believe that a viral infection such as viral meningitis or the common cold sore virus - herpes simplex - causes the disorder. They believe that the facial nerve swells and becomes inflamed in reaction to the infection, causing pressure within the fallopian canal and leading to an infarction, the death of nerve cells due to insufficient blood and oxygen supply. In some mild cases, there is damage only to the myelin sheath of the nerve and recovery is rapid. The myelin sheath is the fatty covering-that insulates-the nerve fibers in the brain.
The disorder has also been associated with influenza or a flu-like illness, headaches, chronic middle ear infection, high blood pressure, diabetes, sarcoidosis, tumors, Lyme disease, and trauma such as skull fracture or facial injury.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.