A form consists of two parts in Lazarus (and in Delphi): a .pas file with the Pascal code, and an .lfm file with the streamed properties of the components on this form. Usually you do not see both on your monitor because the source editor overlaps the form in the form designer. Press F12 to toggle between both views.
If the form still is not seen it could be possible that the original developer of this project had two monitors and used the second one to design the form. As a consequence the Left property of this form is quite large, larger than what a single monitor can display usually. Therefore, you should keep dseligo's trick in mind how to "center a lost window".