down one line?
You have inadvertently typed "C-x C-n" (set-goal-column) which sets the "goal column" to the column where the cursor was. To undo this type "C-u C-x C-n".
If you make this mistake frequently, you might want to unbind or disable this command by doing one of these two:
(define-key ctl-x-map "\C-n" nil) (put 'set-goal-column 'disabled t)