hello,
this is my first post so apologies in advance.
basically, the rundown is: I recorded a 2-hour long audio with my friend in march 2020. from march to august, I edited the audio file with ease and had no problems with audacity or opening the file. I have been unable to work on the project until now. so, I opened my laptop to work on the file (which, before you ask, has never been without its _data folder to my knowledge ).
when I clicked on the project file to open it, audacity launched before giving me an error message that basically said: “Error Importing: aup is an Audacity Project File. Use the File > Open command.”
I received this error message even after using the File > Open command and am growing more and more nervous that this file is irrevocably damaged (as audacity’s FAQ page seems to believe.)
some additional information is that when listed on my file explorer, the audio file still has data, so I do not believe it only contains NULL data.
also, when i check the properties > details tab, there is still correct information about the file, leaving me to question if it is corrupted or not.
any help would be greatly appreciated 
The AUP file is a text file and will open in a text editor such as Notepad or TextEdit. If you do that (don’t save anything), you should get pages of English-like programming language such as this.
Here’s one I cleaned up a little bit by adjusting spacing on the screen. The red box is the show name burned into the programming. If you look carefully, you can see the same thing in line three of the top example. Both are valid.
This bottom example also illustrates the three sections of an AUP file. The top bunch is the header with general show information, the middle bunch has to do with Left stereo sound and the bottom bunch for Right.
If your AUP file has graphics characters, black blocks, or NUL NUL NUL NUL, then it’s gone.
Did you Export a WAV (Microsoft) 16-bit sound file or even an MP3 sound file anywhere in the process? That’s your way out.
The _DATA folder has directories and stacks and stacks of little six-second AU sound files. Each one of them will open and play in Audacity, but without the AUP file, there’s no way to put the show back together—particularly if it’s an edit.
What does your AUP file look like?
Koz
I recorded a 2-hour long audio with my friend in march 2020.
Which Audacity are you using? Help > About.
Koz