So, I have created a simple dungeon following the layout tutorial on the Creation Kit website and all was running smoothly until it came to connecting it to the outside world. The two load doors are linked and everything, exactly how the tutorial showed me how to. I also followed YouTube tutorials. The problem is that the interior load door just decides not to load, it is a clear view into the void and I have tried everything to attempt to make it show. Once you get into the dungeon, it won't let you leave via the door because, well, it isn't there. I don't understand what I have done wrong and I have repeated the process a couple of times to make sure I didn't skip anything but it is always the same outcome. The exterior door is perfectly fine.
This wouldn't bother me so much if it didn't have anything to do with my Year 12 Research Project, but alas if I can't do this then I will fail Year 12 >.< I would appreciate any help asap.

You've checked that the door isn't disabled? What base object is it, and if you create a new reference of that base object, does that one then show?

Also bear in mind that doors are persistent references, and therefore persist in saves. If you had the door in a previous location, saved, moved the door then re-loaded that save, then the door will still be in the old location on that save.

Persistant saves are a bane. Even for clutter. I moved some objects around and when I entered the game they kept showing up in the old locations. For working on a mod always have a save that is before you even started the mod for best results of checking on your progress.