My nether portals seem to have disappeared and I have no idea how to fix it.
My first nether portal spawned above a lava lake, because of Minecraft building a row of blocks around it I was able to build a bridge to the nearest piece of land and explore from there. I stopped playing for a few weeks and when I tried to enter it today it dropped me straight into lava. I went into Creative mode to see if I can tell why I keep falling/where I can place blocks to stop me from falling. But there's no portal? It seems like I'm falling from nowhere in to the lake and upon exploring around the lake I can't see any of my previous build
A had a second portal that was inside the nether bed in the nether but when travelling through that portal I suffocated. I dug around later and couldn't see any portal I could have come through? I was just in a pocket of rock?
Has anyone experienced this? How do you get the portal to show up?
I don't know if this will help but my seed is -5030493358026809513 and my overworld coordinated for the first portal is around x: 454 z:-337
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.
What GeoFor said, I have had similar things happen to me before. Rebuild your portals on both sides, or if you don't have a diamond pickaxe, use TNT to blow up the portal itself and then relight it with a flint-and-steel.
I hope you managed to change to creative mode before losing all your stuff!
When I'm not sure, I sometimes drop my stuff in a chest next to a portal (like how ruined portals are) and go in bare handed just in case, and if it's fine, I go back and get my stuff.
As of 1.16 snap 06, portal transitions are nearly seamless which makes this easier (and faster!).
Even if something caused the chunks in the Nether to regenerate the game should have recreated the portals - I delete region files all the time when testing mods, even from my Survival worlds once I no longer need the Nether, and I've never had any issues with portals not regenerating - after all, it is just like the first time you go to the Nether - the game searches for a portal and creates one if it doesn't exist, same for returning to the Overworld.
Unless they were reset while you were still in the Nether since portals are only created when the player travels between dimensions (newer versions have numerous chunk corruption issues - I wouldn't play on them even if that was the only option). I've also had the game crash immediately after entering the Nether and no portal was present when I reloaded the world (it doesn't remember that you had just teleported and it hadn't made a portal yet, or it didn't save the chunks) but you didn't mention anything about crashes and only experienced issues after traveling between dimensions (which as previously mentioned should have recreated a portal if one didn't exist or the chunks were reset).
Also, I found this bug report, marked as "cannot reproduce" but without a clear resolution:
This related report, which is at least one cause of the previous issue, is still marked as unresolved with versions up to 1.16 snapshots listed; have you checked your coordinates in F3 after teleporting and not finding a portal? They may match the Overworld coordinates, instead of being divided by 8 (conversely, going from the Nether to End might not multiply them by 8):
Notably, while I've never experienced these issues in 1.6.4 the game does generate a chunk or two at the exact coordinates I entered a portal, even if it has always correctly sent me to the correct location and I've never had weird issues like being set on fire or taking damage (for example; the End portal is centered at -775, 528 and the End itself has a couple chunks there, far away from the main island).
Man your posts are so long and detailed, I'm not sure whether to be in awe or annoyed. I don't know if a long answer really helps someone who wants a quick fix or who isn't dealing with a startup /load world issue.