If you closed Audacity on a relatively long show and then closed the lid right away, you may have crashed your show. I don’t think Audacity is all that obvious when it’s busy saving a show. You could have closed the lid on part of the save.
Were you trying to open the project by using drag&drop to drag the project’s .aup file onto an open empty Audacity project?
a) You can import audio files into an open project like that
b) you can drag an audio file onto your Audacity app icon - and that will open Audacity and import the audio
c) you can drag an Audacity project file (projectname.aup) onto your Audacity app icon - and that will open Audacity and that project
d) You cannot drag an Audacity project file (projectname.aup) into an open project - even an empty one.
There is a slight argument for allowing d) to work when the project is empty - but if it were not empty it would need to open a new project. Audacity cannot merge projects - allowing d) could easily encourage users to think that it could or should.
Or, in different words, that’s the end of the show. The AUP is, or was the Project Manager File. That tells Audacity what to do with all that stuff in the _DATA folder. If you were saving an edit, the stuff in the _DATA folder is scrambled. Permanently.
It is recommended that you Export a WAV (Microsoft) 16-bit sound file of your work at critical times. A WAV is a stable, stand-alone sound file that plays anywhere and can be edited. What a WAV can’t do is save multiple timelines, offsets, labels, and other editing information. That’s where you can use Projects.