You're not screwed - you had an anxiety attack. It happens to lots of people. Your instructor knows that you know the information, or he wouldn't be giving you the time to provide it to him. Look around MT, ask for help - we don't do front stance and fighting stance, and I'm not sure how they relate to walking and L-stances, our closest equivalent, so I'm not the right person to ask - but some people just don't respond well in public/high stress settings. Don't give up now - if that's all you need to do, then you're all set - just write a description of each stance, including how long it is, how wide it is, what part of the foot and/or leg the length and width are measured from, the weight distribution (e.g. evenly on both legs, more on the front or back, proportions if you have them), and (if you know it) the types of techniques done from each stance, and give it to him. If your instructor didn't think you already knew the information and choked in a high stress situation, he wouldn't be giving you this second chance to demonstrate your understanding - so don't give up, demonstrate!
I'll give you an example: my class tested 2 weeks ago. Two of my students missed their breaks - both young teens (one 13, one 14). The 14 year-old, a very sensitive young girl, was so upset she was in tears - but by the next time class was held, she was calm, in charge of herself (testing had been 3 1/2 hours, and as senior, her breaks were last - it was 9:30 at night on a school night - she was tired) - and she made her breaks, and was promoted - because she overcame her difficulty and proved she knew what she was supposed to know, in this case, how to apply the technique. The same went for the 13 year-old - he missed his hand break, came to the next class, and is now a green belt - and he's on medication for anxiety and other emotional concerns; when he didn't know what to do at his high yellow testing, because he was told to do something he hadn't learned yet, he got so upset he broke out in tears and walked off the floor - but he came back, did his best, and passed.
Another example: the senior member of our association was testing for his VII Dan, and stepped with the wrong foot for his testing pattern, Tong-Il - the performance of which was his only testing requirement. He looked at the testing board, apologized, and asked to start over, which they told him to do - and he then completed the pattern correctly.
Mistakes happen - it's how you deal with them that determines your mettle, not whether or not they happen. Get to writing, post it here for those who use your stances to provide feedback if you feel you need it, and give it to your instructor - and quit stewing about it; nothing you can do will change the past, but you can change the future - so get to it!
