I wrote the following command:
In[1] := GraphPlot[{a -> c, a -> d, c -> e, c -> g, e -> h, e -> g, j -> g, g -> b, h -> g, d -> f, d -> h, f -> k, f -> i, i -> h, i -> d, i -> b}, DirectedEdges -> True, VertexLabeling -> True];
The first time it worked, after that I keep getting the same error.