If you turn a monitor off, your computer should stop detecting it, and the other monitor should become the primary monitor. In the current version of Windows, it should remember your "profiles" so when you turn the monitor back on, it should revert to where it was.
That said, I use a laptop with multiple monitors and docking stations in multiple locations. I move around quite a bit, and Windows will get confused about monitor orientations, resolutions, etc, and it can take a few reboots to get it back to where I want it to be. The world is an imperfect place.