Why my edit mode crushed?

Hello to everyone, i am in the process of studying and i am in the part of game editor. Its been a couple of time that my progress crushed and i had to fix everything from the start again. Now it happened again and i have to fix it again from the start. Can you please help me uncrush it and can someone explain me why it crushes or is there something i am doing wrong and i lock it?? I follow step by step the rules of the video. But if i go further and it crushed again it would be very difficult to start all over again. unity version 4.5.4f1