injury to occipital part of head can many times lead to loss of sensation of smell also known as anosmia. many times it may recover gradually or sometimes it may be permanent.
the cause of anosmia is due to injury to nerves which are responsible for carrying sense of smell to brain. if there is just injury not severing bid nerves it will recover. if nerves are completely severed then I fear the condition may be permanent
you need to consult neurologist rather than ENT specialist for this condition