Mathematica notebook opening off the screen

Yes, I have encountered this problem very often. For me, the reason is that I have 2 screens on my desktop pc and when I save a notebook which is on the second screen and open this notebook later, e.g. on my MacBook, then it happens that the notebook window has coordinates outside my Mac screen-area.

The solution is simple: Assuming testme.nb is your notebook (you need to include the path as well), then you can do:

nb = NotebookOpen["testme.nb"];
SetOptions[nb, WindowMargins -> {{0, 0}, {0, 0}}]

and it should jump to the upper left corner of your screen.

In Windows, it is simpler to right-click the Mathematica icon on the taskbar, select the notebook, which is still off-screen, then hit Win-LeftArrow (or similar combinations) which will dock the notebook on the left of the visible screen.