So mediation was refused by the NHF or you went to the session and weren't able to reach an agreed settlement. Now you have to decide on what your options are and how you might solve the problem (whatever the problem is). If you decide that it must be resolved you are probably going to have to get outside help by way of local government for planning issues and some noise problems, or contacting the local land board regarding fence disputes, the police for late night noise or combinations of all these organisations and, hopefully as a last resort, legal advice.
So before you charge off to employ a lawyer or just go ahead and build the boundary fence without an agreement STOP, ASSES THE SITUATION and decide what your next move should be, you need a strategy, a plan, most of all you need to inform yourself as best you can as to your rights and obligations according to the law in your state regarding your issue/s and consider the options (if any) that you have. At this point I will refer you back to the menu listings to have a look at the various categories of NFH problems and the various avenues available to seek resolution.