I've successfully duplicated your symptoms on Office for Mac (it's an Office problem, not a Windows problem):
See the rulers at the top? Notice that the small rectangle below the rulers (the block thingy below the hourglass-like arrows) is to the left of the left edge of the text box. This basically means that the text will start off of the screen, outside of the text box.
How do we fix it? Let's move the boundary back into the text box. I'll drag the rectangle to the right until the left margin is within the box again (don't drag the hourglass arrows; those control the newline and line break points, which you can move all at once with the rectangle). If you can't see the rulers, go the View tab in the Ribbon and click Show Rulers or something like that. Once I finish dragging the rectangle marker back into the correct place, the text appears normal again:
Well would you look at that! The text is back in the box, right where it should be.