Portions of the world disappears

I have just lost 3 weeks of progress on a tower mall that I was contructing. In total about over 900,000 blocks placed within the process. 

I experienced chunk loss in the flat world that I was building in. I was confused but I was fine with it. I thought it was a minor single chunk glitch but in the same day as I found the chunk lost. It happened.

I rejoined the world and though halfway of the chunk loading process; I accidentally exited the game. I rejoined but it crashed. This is normal for me but what was abnormal was the fact that I entered the world immidietely as I clicked the game icon. I had not noticed any changes so I continued building. I continued placing multiple blocks in a rows then that was when I realised that everything, other than the part of the build that I was looking at, was lost. My towers, and a long portion of the mall completely disappeared. Leaving the flat grass area empty as if someone /fill the entire thing.

My build that was left is chunky and I can visually see the chunks they were in. It is a sad sight. I could not imagine that this is what I had recieved after working many days of this build.

I wish to have this issue fixed as soon as possible so I do not lose any other progress from my other builds. As of now, I will not play any of my other worlds in fear of sudden issues like this poping up once more.

Please respond quickly and I hope that this will be fixed.