Unless you actively changed the project's path when you created it, the project should be in your Documents folder (that's the default location). When you start Tropy, it should try to open your most recent projects (if it can't open them for some reason, e.g., because the file was moved) it removes it from its list of known projects and tries the next one (or displays the new project window).
You can open your project by selecting
File -> Open from the menu in the project window (if Tropy opened a different project) or in the new project window. If you're on Linux or Windows, the menubar is not visible in the new project window, but it's there, and you can make it appear by pressing the Alt key. Alternatively, you can also you the shortcut
Ctr+O or, on macOS,
Cmd+O to open an existing project. In the file picker, navigate to your Documents folder; your project should be there.