Crashing and or blocks are missing and left blank. Plus patches of colored and textured block randomly appear.

Game has crashed and struggled to reload. At random times and places patches of brightly colored blocks appeared. When I tried to click on one, they disappeared and I crashed again. While exploring I came across various areas where blocks were missing or only just one side of them showed allowing me to "see through" the land to the sky beyond. In some cases I could see tunnels that were lit that I had dug. I did not attempt to step on those blocks. I love my game and use it often. I would hate to lose my worlds. I only have 5, but they are expanded on regularly. One is very large and represents months of work. I'm afraid if I keeps trying to play, I will lose these world as I have read that others did. (Maybe I got it wrong, I'm actually a 63 year old newbie!) thank you for your time, I don't know what I should do.