To fix this what you need to do is change your window settings. To do this hit [WINDOW] (right next to Y=) and change the settings of Xmin, Xmax, Ymin, and Ymax (make sure the value of Xmin is less than the value of Xmax, and the value of Ymin is less than the value of Ymax). If you prefer a simpler way, hit [ZOOM] (right next to Y=) and hit  to change your window back to the standard setting.
If these changes do not help, then you need to reset your memory. (to do that hit [2nd] ,[+], , , , [ENTER])