(Press Windows Key + R, paste this command line there, click the Run button.)
If you find it, please rename massdown.bak to massdown.dat, then start Mass Doanloader.
This will open the history panel.
If it is filled, you may use Mass Downloader File | Import | URLs from file... and open the Hystory.dat file from the same APPDATA folder. This may help to retrieve links, at least.