Did you try a search for the file? Or for all .ppr-files?
(Do you know how to search for files on a computer?

)
Always save your projectfile in the same folder as your project.
If you can't find it, but you have the project opened: press CTRL+SHIFT+S (or select Save as...) it will open the last location you used to save a project (unless that location is disconnected).
This way you can find it if you don't know how it's named.
My folder structure (I don't use these names: the names are there to make clear what it is):
- ProjectFolder
- - projectfile (not a folder, but the file!)
- - VideoFootageFolder
--- - VideoDataDay1Folder (or MemoryCard1)
--- - VideoDataDay2Folder (and so on)
- - SoundFolder
--- - MusicFolder
--- - SoundFXFolder
- - ImagesFolder
- - ExportsFolder
- - MiscFolder
This way you can always find your stuff.
Datamanagement is an important part of your workflow.