"Couldn't find the project data file"

I’m using Audacity 2.3.3
I saved a project I was working on a few days ago. I saved it as an audacity project as I was going to come back to it later. Now I’m trying to open it (the AUP. file) it says “couldn’t find the project data file”. I can’t see the data files there myself. And I have tried searching.
However, around the same time (probably the same day) I also saved another project in, to my memory, the exact same way. When I go to open that AUP. file, it opens with no problem, despite the fact that I cannot see the corresponding data file for it either.
Please help! I’ve been entrusted to edit these files and need to get it done ASAP. I would be so so appreciative of any advice.

An Audacity project consists of multiple files. For example, if the project base name is “foo”, then there will be a “foo.AUP” project file, plus an adjacent subfolder “foo_data” containing several other files.

If you move the project file, you have to move the subfolder with it. If you copy or rename either one outside of Audacity, that will likely cause problems. Have you done any of that?

You can try turning on the options in Windows/File Explorer to show hidden files. The exact UI path varies with the version of Windows, but most likely, click the “View” tab on the ribbon, then the “Options” button at the far right. In the resulting dialog window, click that “View” tab. In the list of options, enable anything that says “Show” and disable anything that says “Hide”. Once that’s done, look at your Audacity project that’s working, and compare with the one that isn’t working. Maybe you can spot the difference.