I am on Mac OSX 10.8.5 using Minecraft unmodded 1.7.4 and launcher 1.3.8. Yesterday I created a Nether World portal for the first time and entered the Nether World successfully. I went backwards and forwards between the two worlds successfully for a couple of times and then played around in the Nether World for about 20 mins. I went to go back through the portal and my computer froze for about 10 seconds and then Minecraft crashed. Initially I could not relaunch Minecraft so I decided to restart my Mac. I then was able to restart Minecraft but upon entering my World I was back in the Nether World with no portal and Huge holes in the floor of the Nether World (almost like a UI issue) and none of the mobs paid me any attention. I could not interact with the mobs at all. I now stuck in the Nether World.
Assuming you don't have Obsidian to make a new portal, jump into lava and when you die you should respawn in the overworld.
Once back in the overworld, assuming everything there looks okay, you should save/quit and make a backup of your save. Then you can try re-entering the Nether and see if it's okay. If it's still messed up, quit the game and delete your save, then restore the save from your backup and delete the DIM-1 folder in your save. That is the Nether. Next time you re-enter the Nether a new Nether will be generated.
You could also just try deleting the DIM-1 folder from your current save. Not sure what happens if you do that while your character is still in the Nether in that save, so save/quit and make a backup first.
How can I troubleshoot and fix this issue?
Thanks!
Once back in the overworld, assuming everything there looks okay, you should save/quit and make a backup of your save. Then you can try re-entering the Nether and see if it's okay. If it's still messed up, quit the game and delete your save, then restore the save from your backup and delete the DIM-1 folder in your save. That is the Nether. Next time you re-enter the Nether a new Nether will be generated.
You could also just try deleting the DIM-1 folder from your current save. Not sure what happens if you do that while your character is still in the Nether in that save, so save/quit and make a backup first.