I have shifted by Dev Env recently to Eclipse Helios on Windows 7. After that the Ctrl + Space Content Assist feature is not working. I found out that that key option is utilize
In my case nothing happens after pressing Ctrl+Space. Right now I have set a different short cut for the same and solved
Check your "Keys" preferences: maybe the Content Assist" shortcut has been overridden by another command which would also use Ctrl+Space.
That could happen with the installation of a new plugin, coming with its own set of commands and shortcuts: the bug 303894 for the XText plugin is a good example.