Try resetting it...
Tools->Options->Text Editor->C# (or All Languages)->General and enable Auto List Members and Parameter Information in right hand side pane.
If that doesn't work try this below...
Tools->Import and Export settings->Reset all settings.
EDIT
1: Close all the tabs and open your file again.
2: Clean the Build > Close the Solution > Restart Visual Studio > Open the Solution again
3: Goto: Edit > IntelliSense > Refresh Local Cache
4: Close Visual Studio 2012 and delete this folder: %AppData%\Microsoft\VisualStudio\11.0\ReflectedSchemas