This generally has happened for me subsequent to a command-line initiated build. Upon opening the IDE I have no intellisense support for the simplest of types.
To fix this issue consider deleting the .projdata file, this is a hidden file found in the \Debug folder of the project:
Also note for ASP.NET applications this file is found in:
C:\documents and settings\[user]\VSWebCache\TestApp\obj\Debug\TestApp.projdata
"You see things; and you say 'Why?' But I dream things that never were; and I say 'Why not?' " - George Bernard Shaw