There could be many reasons why your cat is losing hair around their neck, it could be something relatively minor, like irritation from its collar, or it could be a reaction to its diet. In some instances, it may be a sign of a more severe illness. What to do will vary based on the problem - so if it's your cat's collar, for example, try switching to a different one. Or if your cat has fleas, that could be the problem too. If their diet has changed, then speak to your veterinarian, and change it back to see if the issues with hair loss persist. Of course, you should always seek medical advice if you have concerns about cat hair loss, so you should make an appointment with your veterinarian if you aren't sure of the cause.