Recently, when creating an assembly with 100+ parts, I dragged a motor assembly into the main assembly. Shortly after this, a few parts of the assembly became un-selectable and it seems like the appearance of many other parts within the assembly changed. Since then I have not been able to drag in any parts related to that motor assembly that seemed to break the entire thing. All of the files for the whole assembly are located in the same file but if I try to drag new parts in, they only appear in the file tree but not in the actual assembly and are completely invisible and unelectable. 

Any help is appreciated.