I was working with a part file. After finishing I saved it. Then I need to draw another one. But the second one is similar with the first. So, I made "save as" the first file and rename it. Then I work on the second file. I dragged some tree to hide and drew new. After finishing the second file, I assemble first and second one, everything was fine. But suddenly, most of my drawing of the second part from the tree section got faded, which I can't turned back on. Features and sketches all are present but not visible. What should I do?