Blocks disappear after closing game

I spend 5-6 hours building. I being a novice, I utilize YouTube videos to help with my buildings.
This happened tonight:
I opened the game and decided to build a blacksmith. I found a tutorial on YouTube and used this blueprint. I went from watching the video to opening minecraft and resuming my game to complete what I just viewed.
Three times upon returning to the game, parts from my building were gone, disappeared. I had to replace my roof twice.
This has been occurring over the last few days; 3-4 days.