Just receiving a letter without more is not something the Division on Civil Rights or the Courts are likely to act on. If the landlord had failed to renew the lease when the old one expired, then you likely have a claim. Perhaps your strongest weapon would be the NJ Anti-Ecviction Statue, which makes it very, very difficult for a landlord to terminate someone's tenancy unless the termination fits into a narrow set of exceptions.
Obviously, I don't know all the facts, so how you might best protect your rights could be different. You should talk directly with an attorney to get complete and accurate advice.