You'll need to move your database from where it is located. The problem with where it is located is that it's stored in a location that can get uninstalled/deleted if you upgrade your version of TntMPD.
You have a few options. You could search your hard drive for your TntMPD database. Under the "File" menu click on "Search for Databases" to search for files matching the pattern ".mpd". Once you find it, move it to your Documents or Desktop directory. Then open it from there.
Another approach is to restore a backup. Under the "File" menu, click on "Restore". Find your most recent backup and right-click on it and "Save As". Then choose a location in your Documents or Desktop directory. Then open it from there.