Please use "Pack and GO" while saving the assembly so that all the components/sub assemblies are saved along with assembly and you can easily open the assembly with out any problem.
While opening an assembly the system first check the location of components/sub assemblies to pick the same and rebuild the assembly. If we later rename part drawing which is used for assembly it will effect while opening the assembly next time.
So please please check
1. Whether renamed any part drawing/sub assembly
2. All the components /sub assemblies are available in the location