The window has been scrolled left (scroll-left) for some reason.
C-x < or C-<next> can get you into this state.
C-x > or C-<prior> can get you out of this state.
Note those commands are blocked by default. When using them the first time, you will be asked, whether or not to enable them, in the minibuffer.
Note: if you press two times C-x &...
Just adding to answer by @jue above.
The C-x < ("scroll-left") command horizontally scrolls all text lines to the left, which means that the window's view "pans" to the right. The emacs doc section on Horizontal-Scrolling does a pretty good job of explaining.
The way to get out of this pickle is as follows:
Get back to start-of-line ...