If your pastry cream didn't thicken up, there are two potential reasons that are the most likely cause. The first is just forgetting to add cornstarch. If that happens to you, you could save your pastry cream by whisking the cornstarch with a few additional tablespoons of milk to create a slurry, then whisking it in and cooking until the pastry cream is thick. Or the other reason could be just not cooking the pastry cream long enough.