Welcome back!
What level is your characterthat you are trying to get to Khaz Modan on?
You need to be level 70. Maybe you are on a level 70, but posting on a 51?
If you log in on a level 70 in Stormwind, Orgrimmar, or Valdrakken (and probably other places) you should get the starting quest for The War Within pushed to you on a big “What’s New” splash screen that you absolutely can’t miss.
If you are on a 70 that did not get the quest, please post the name and realm. Is it a 70 that you boosted, or did you level it?