There are a number of possible reasons:
(1) If you are getting an error you may have another operation running. For example, in a TI-8x calculator, plotting a stat plot can often interfere with graphing.
(2) If you are trying to graph a trigonometric function and you are in degree mode instead of radian mode, the graph will not be as expected and may not appear in the viewing screen,
(3) If the graph lies outside the viewing screen you will not see anything. Try looking at the table to get an idea of the function outputs and resize the window.
(4) If the function is highly iterative or requires a great deal of computation (this happens in polar graphs quite often) the calculator can take minutes to compute. In a TI-8x you will see vertical segments in the upper right corner indicating that the calculator is processing.
There are other possibilities, but these are what I look for first.
-check viewing screen