I had a similar experience on my first memo 1L year. I ended up meeting with the prof. enough that I got a better idea of what he was looking for. My 2nd memo was a big improvement and I ended up with a B for the semester. It was a big relief after I tanked the first memo. See if you can get some time with the professor, make sure you take notes of what he/she is looking for. No reason to panic. You'll turn it around.