There is no one good window range for all graphs, since graphs can have different areas of interest.
Press the ZOOM key, then select ZStandard. This will restore the window settings to the default range, which will at least allow the graph to be displayed. You can then change the window range and/or zoom as desired.
If this doesn't do it, please reply to this post, giving the function(s) you're trying to graph and your current window settings.