There are a number of causes of facial weakness, Bell's palsy being the most common. (Bell's palsy is also called 'idiopathic' palsy which by definition implies that the cause is unknown).
Other causes include Ramsay-Hunt syndrome which is facial weakness caused by the chicken pox virus (Herpes Zoster) which usually responds quite well to medications (in fact it is thought that many patients with Bell's palsy have Ramsay-Hunt syndrome so are treated as such). Middle ear infections (otitis media) can also cause facial weakness. Very rarely conditions such as multiple sclerosis, tumours of the parotid salivary gland and sarcoidosis can also cause facial weakness.
Most people that have facial or Bell's palsy recover . This can take a few days to a few weeks or sometimes months.
'Patient UK' provides useful information on this.
Facial Palsy - Patient UK