According to Wikipedia, brain freeze is the acute cognitive faltering due to stress. (Of course, there is the other commonly used definition of a form of brief cranial pain or headache commonly associated with consumption (particularly quick consumption) of cold beverages or foods such as ice cream and ice pops.
What do you do when your brain freezes and you find yourself sitting and staring at your keyboard with no words coming to mind? One blogger, Chris Abraham recommends that you just write about one of the many ideas floating around in your head and don't worry about being perfect.
Purdue's Online Writing Lab, has an article by Sean Conrey who recommends that you try starting in the middle of your story and worry about the introduction later. No one will know or care if you didn't start at the beginning.
In a blog, Love and Laughter by Peri Kinder, she states that maybe you should just admit you are a terrible writer and go get a job at your local department store. However, I will take that one step farther and say just go and stand in line to apply at the local department store and you will all of a sudden find that missing inspiration to go back home and write furiously!
Laugh at yourself and relax! None of us have it all figured out. (I assume)