Re-Opening Previously Saved Files, but having them say they were deleted

I had a project I was working on and needed to save it to work on later. So I saved it and put it on my external hard drive, and then when I went to open it to work on again, I received an error message that said “cannot open an empty project. Adding a default empty scene.
-Syntax error at line 1: unrecognized attribute (<)”

I was wondering if anybody else has had this problem before and if there’s a way to solve it or prevent it from happening again.

Did you copy the entire folder? or just the .xstage file?