Try following Things:
>Check whether parts are in change Transparency mode or not. if yes change the mode.
>Sometimes it happen`s in big assemblies and after reloading assembly again its become allright. Try reloading also File>Reload.
Check the above steps ...
neither option helped
Have you tried changing to a different display mode, such as "Hidden Lines Removed", and then changing back?
Have you tried a Ctrl+Q rebuild?
If neither of these work, it might be a graphics card problem. Can you post your computer specs, including graphics card?
Reboot did the trick.
I guess it was an overload for my pc with the backgrond programs.