I occassionally get this problem in NX6.0.5.3 and have asked about it as part of another constraint problem posting here about a year ago, but no response to the blanked component issue.
Frustrating problem...
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
If one in the assembly navigator uses the option "Show only" on a component, the assembly itself becomes hidden.
I have seen cases where new things then gets created hidden.
Tick the box of the assembly itself.