To fix this, follow the two solutions displayed on the following link (otherwise, click more):
“Don’t close Visual Studio but close all the files and open them again by clicking on the file in the solution. The file might have been opened by Source Safe or you maybe opened it from Windows Explorer. In this case the file doesn’t get associated with your solution and the Go to Def. doesn’t work.”
“1 -Close the solution.
2- Delete the intellisense database file for the solution: <name of the solution>.ncb
3- Open the solution.
4- Rebuild the solution.”