Went to the Nether, got lost, and lost my castle!
Solution 1:
You should go into survival mode, turn on keep inventory, go to the nether, and kill yourself. You should NOT sleep while you're lost, or else when you die, you will be sent back to where you've last slept.
If this doesn't work, you could try building another castle. If you don't want to do that, try to create a map and if you don't see your castle on the map, fly somewhere farther away from your portal. You should keep creating new maps and spiraling away from your spawnpoint. You can do this without using maps, too. Eventually you should find your castle.