Nothing, just the .au files. When I try to load the project into Audacity, this is confirmed by a message from the program saying it can’t find the data files.
The usual cause is “user error” (inadvertently deleting the _data folder, or moving the .aup file to a new location without its _data folder).
A failing hard drive could also account for the problem, but that is rare and very likely to cause other problems (files disappearing or becoming corrupted).
Doesn’t Windows have a Restore function that can step you back to an earlier time?
If you have the opportunity, Export the work as WAV (Microsoft). Use only “safe” characters in the filenames (upper, lower, -dash- and underscore) and don’t export any stereo work over about three hours (six hours for mono).
This isn’t going to work if you have multiple hours long projects or multiple tracks (anything over stereo). But WAV files are very robust and not subject to system coordination difficulties or odd interactions between files and folders.
If you are recording live, it is strongly recommended you Export the recording as WAV (Microsoft) backup before you do anything to it. Move it to a safe place.