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A1: You forgot to specify a range on the independent variable. 

 > f:=2-x;
 > plot(f);              WRONG
 Plotting error, empty plot
 > plot(f,x=-1..1);      RIGHT

A2: You might be plotting a quantity that isn't defined. For example, you might have defined Y but then asked Maple to plot y. Maple is case-sensitive, and many strange behaviors are the result of typing a variable name in the wrong case.

Another way a plotted quantity can be undefined is if you didn't use the assignment operator := to define it. 

 > y=sin(x);              WRONG
 > plot(y,x=-1..1);
 Plotting error, empty plot

A3: There might be a scaling problem, as in this example. 

 > plot(exp(x^2),x=0..10);
Here the numbers get so big that all of the points Maple calculates before the last one are negligible compared to the last point in the plot, so they are all lost in the x-axis and no curve appears.

A4: The quantity you are trying to plot might contain variables other than the independent variable in the plot. 

 > g:=x^2-3*a;
                                        2
                                  g := x  - 3 a

 > plot(g,x=-1..1);
 Plotting error, empty plot.
Here both a and x are variables in the definition of g. In order to plot a function Maple needs to compute numerical values for it at different values of x, and that's impossible until all of the other quantities in the function have been given numerical values. In this example, assigning a value to "a" fixes the problem. 
 > a:=1;
 > plot(g,x=-1..1);          Now this command works.

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