This sort of thing happens frequently with people in head on car crashes when their heads hit the windshield or dashboard. Either there is damage to an epithelial layer of odour-sensing cells, or the nerve connecting these cells to part of the brain is broken or disturbed. You need to go see a good neurologist.
For more information, search the 'Net for "anosmia", "loss of smell", "olfaction" etc. There's a lot out there. For an older but still relevant literature review on olfaction, see Lancet Ann Rev Neurosci 9:329-355 (1986). Also: Mombaerts Ann, Rev Neurosci 22: 487-509. Also check http://www-surgery.ucsd.edu/ent/davidson/NDC/booklet.htm http://health.ucsd.edu/health_info/smell.htm