Hi Frustrated Swede,
Sorry you are having a frustrating experience!
If you installed the new version of PingPlotter over the top of the one having issues than your data should be there. If it's not automatically showing up than you might have to do a little looking around for it. The first place to look is in the Sessions Manager (File -> Manage Sessions). If your old data files are there (and they should be), the Session Manager will search and load them up for you. If they don't show up in the Sessions Manager than either they are likely stored in a different location than the default, PingPlotter is somehow not recognizing the files, or they’ve somehow been deleted.
If you manually removed the old install (Add/Remove Programs) and didn't uncheck the box that says "Remove all PingPlotter settings, directories, and log files" (which removes everything) then that will indeed delete your data files.
If you are still having trouble locating your old data you can always manually check on your data files. Depending on if you are running PingPlotter as a service or not - your data should reside in one of two places on your computer. I am going to assume that you are running PingPlotter on a PC (locations will be different on a Mac). If you installed PingPlotter to run as a service (It should say "Connected to Service" in green in the upper right side) then your data will reside in %programdata%\Pingplotter 5\SampleSets. If you are just running it as a local app only, your data will be in %localappdata%\Pingplotter 5\SampleSets. You can enter these locations in a Windows Explorer (WIN + E) location bar and it will take you directly there.
If you have any other questions let us know.
Good luck and let us know if you are able to find your data!