A number of players have reported that skins are not showing up either on the website or in game. I'm currently experiencing the same issue, so it is not isolated to you. My skin appears fine in single player 1.12 vanilla, but I appear as Steve in Hexxit. Skins were also showing as standard Steve skin on Enjin yesterday so it isn't anything directly related to the server or Hexxit that I can tell. Mojang shows skin services as online and healthy. My only advice is to be patient, the skins should return. Changing your skin will most likely NOT fix the issue.