Set the pan back over medium heat. Whisk constantly.
The pastry cream will start to look thin and frothy, but eventually it'll start to thicken after a couple of minutes. Once its thickened to a pudding consistency, turn off the heat immediately. If your heat is too high, or if you don't whisk consistently, you might break your custard. If that happens, see notes above to see how to fix a broken pastry cream.