Dear people from the future! Here's what I learned:
Disabling hyperthreading and OCing my i72600k to 4ghz is the only change I made between this game not loading exteriors, and suddenly loading them again. I have no idea if this really helped, or if the game just chose a random time to start working again. It's a Bethesda game, so who can say? Well worth reading down for extra suggestions such as console commands and waiting for scripts to reset.

Original Post follows:

I have no idea how this happened. This morning I was playing the game without any issues at all. Now, I can only access save games that are saved in interiors. Loading an exterior save, or moving from an interior cell to the exterior worldspace leads to a loading screen that never ends. The music keeps playing, and the tool tips keep changing, but it never loads.

I used the search feature first and found numerous similar threads, but none of them had found a solution. Has anyone worked out what causes this yet? I haven't installed any new mods, changed any graphic settings, altered the ini files, or anything of the sort. It just... stopped working. For no reason. Unless there's been a new Steam patch... It doesn't make sense!

Things I've tried:

Reloading earlier saves.
Running the game without mods.
Regenerating ini files.
Loading from another character who IS in an exterior.

Nothing works. For some reason, the save file for this character has broken, preventing me from accessing the exteriors. It's even affected all of the older saves for this character, which is completely bizarre, since I had been running around in the world after those saves for a while. It's a game breaking bug without any logic behind it, as far as I can see.

Has anyone else had this issue? Anyone found a solution? Deleting this character would be painful given how much time I've invested...

have you ever played with the ugridstoload setting?

IF so.. some cases when setting it lower either results in a crash to desktop OR an infinite loading screen.

however this doesn't make sense since your able to load into an interior zone and only appears to effect your exterior..

I'm not sure what to suggest.

I have it set to 9, yes. I guess that could be the cause, at a long shot, but it doesn't affect my other characters. My longest running character has over 100 hours to his name, more than double what the problematic character has. His save file isn't affected. New games aren't affected either. Just this one...

Is there a console command to teleport my character elsewhere? She's in... um... a village just east of Markarth at the moment. Save file is in a house there, and exiting the house leads to that infinite loading screen. Would I be able to teleport to, say, Dragonsreach? I'd like to try entering the exterior worldspace from a different location.

"coc" is the command..

"coc whiterun" .......or the name of a hold of your choice...

Try waiting 73 hours before going anywhere to clear out any scripts that may be causing this issue.

It's working! Turned on the game with the intention of trying the above two suggestions, but tried walking through the door leading outside first and it worked!

The scary thing is, that the only changes I've made since my last dozen attempts are that I disabled hyperthreading on my i7 2600k, and overclocked it to 4ghz. So either the game spontaneously started working again, just as randomly as it had decided to stop working (which actually wouldn't surprise me), or disabling hyperthreading is some sort of fix. I doubt the extra .2ghz I just gave it would have made much difference.

Thanks very much for all the help guys. I'll update the OP for the people from the future to benefit.