Unfortunately, what you describe is quite common after a head injury.
This is because the olfactory nerves (the ones you smell with which are thin and small) are located in an area where they travel through small holes in the bottom of the skull. And when people get into a head injury situation, the brain moves quickly within the skull, and this can "shear" those nerves.
So, the loss of smell sensation after a head injury can occur. Hopefully one regains some of this over the next few months, but in some people, the loss of smell is permanent.
This is called post-traumatic anosmia.