There can be an issue with the intellisense .ncb file. If the no compile browser (.ncb) file located at the root of your project directory is read-only or becomes corrupt in some manner, IntelliSense information will no longer be available. To fix this, do the following:
Close the solution.
Delete the .ncb file.
Reopen the solution. Reopening the solution creates a new .ncb file.
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.