Is there any chance you have a P and a p in that ContourPlot? The image looks like that might be.
When I make both p or make both P I see a plot.
When one is p and one is P I see an empty plot.
If that isn't the problem then start up Mathematica, pull in your notebook with just that code and nothing else, evaluate everything just one time, save the notebook and attach that notebook to your reply. With that someone might be able to track down the problem.