Sounds like the DIM-1 file in your world save got corrupted and the game replaced it. You can wipe out all changes since the world started if this happens. The DIM-1 file is the nether map and the DIM1 file is the map of the end.
Some players, for example, remove the DIM1 file on purpose so the game replaces it to allow fighting the dragon again. I suggest destroying your overworld portals and rebuilding them. This should give you back the netherside portals.