Maybe spoilers for newer players. I have been playing a few months, just wondering around, playing solo, very relaxed pace, just doing quests/daily quest just hit level 110 alliance Elf druid, explored Northrend and Pandaria some, I was playing today and now I can’t get back to Darnassus with my hearthstone cause it says it was in ruins. It wasn’t like this before. I don’t know why it suddenly changed.
Without giving away spoilers/specifics, there’s a huge event leading up to the current expansion at level 110 that makes it impossible to visit Teldrassil. If you go to the Darkshore zone there should be a little white speech bubble on the map. An NPC named Zidormi is there and she will take you back in time so that you can revisit Teldrassil and Darnassus again!
The actual events of what happened to Darnasus were part of a limited time event before BfA was released. So, unfortunately, you can’t play through those quests any more.
The good news is that there are two short stories available to read on this site which cover what happened. (One is from the Horde perspective, and one is from the Alliance perspective.)
If you talk to Zidormi & select the “what happened here” dialog option, you can see what happened to Teldrassil & Darnassus. But it doesn’t explain how things got to that point; for that you’ll want to read the stories Udiza linked to. For even more backstory, read the novel ‘Before The Storm’ by Christie Golden.