I dont have any image, but i'm guessing what happened is that your knight was pinned to your king. in chess you are not allowed to move into check. so if you moved the knight and it would put your king in check, it is considered to be an ilegal move.
I was recently playing a chess match with my friend in class, and noticed something funny. My friend tried to move his knight, but the game wouldn't let him - We were on some off brand website called something like 2 player games. We gathered it was probably some unwritten rule of chess, not sure though. The knight in question is on C3 and it's a white knight. Half the board was cut out on the photo as I had to put my phone away quick. Phones aren't allowed out in my school, If anyone knows anything, let me know. Thank you.
-edit, sorry, trying to reupload the chess file as it's not working.