I've had a cat with a wound on her neck, that lost the hair, and got really swollen. We took it to the vet and it was a cyst. The vet, gave us 2 choices lance it and take the cat home with the most definite chance it would just swell up again and eventually poison the cat or choice #2 get an operation and remove the cyst wall and prevent it from ever filling back up. Good luck.
Your cat is most likely suffering from a skin disease such as stud tail or ringworm infection. It will be advisable to take your cat to a vet for a blood test to determine the exact cause of hair loss and source of the wound.