This rather alarming situation has now happened to me twice.
I opened a .pln file (say, "hospital.pln") and the project which appeared was another, ("school.pln"). It would appear that school.pln over-wrote hospital.pln
The 2 files were in different folders, but shared on a network. I had been working on "hospital"; someone else was working on "school", and, when they saved it, it over-wrote my file.
The second time that this occurred, a different file over-wrote my file, but the 2 files were in the same folder. However, in each case it was "hospital.pln" that was over-written.
I'm guessing the problem may be lurking on our network (permissions?) or it might be a problem with the hospital.pln itself. For caution, I have clipped out the content of this project and copied it into a new file.