Welcome to the forum. The most likely reason is the Assembly has lost the references to the Parts and Sub-assemblies. Did you create this Assembly or did you get it from someone else?
Thanks for reply Mr Glenn Schroeder ,i get download this assembly from website
Can you send the link from where you download the files?
Right click on any component and select open. If file open up fine then close that and set all components in the assembly to unsuppressed state.
Click LMB on Top level assembly in Feature manager tree > menu EDIT > then option UNSUPPRESS WITH DEPENDENTS
Other reason can be, that you have message with Internal ID... in the Dismissed messages. Take a look on attached image and clear all messages, then open the assembly.
Nay Linn Did you try it?
Retrieving data ...