There are many possible reasons for feline hair loss.
Three things to consider are:
A. Age. If your cat is getting old then this could be a natural part of the aging process.
But more likely...
B. Stress. Has something happened recently in your household that might have put your cat on edge? Sometimes hair loss can mean that the cat simply is upset about a change, and dropping fur is it's body's way of reacting to the tension.
C. A disease. Sometimes if your cat gets sick, then they'll lose fur. Again, this is sort of an instinctual reaction type of thing.
Over all, I'd recommend taking your cat to the vet. That way you can get a professional opinion.