I can't say for sure what the problem with your app is, but I've had this symptom when I've modified my ASPX/ASCX xhtml while the application has been running.
Consequently the generated designer.aspx.cs/ascx.cs file couldn't regenerate with the added/modified controls. So when I've gone to compile, the compiler has complained that the controls were missing.
When I've added them manually to my handwritten aspx.cs file and compiled it's caused a clash as the designer.ascx.cs was subsequently regenerated and now I've got duplicate conflicting declarations.
This issue has also caused the intellisense to stop working.
To solve the problem I had to:
- Make sure the application was stopped - make sure the ASP.NET Development Server is stopped too, just for good measure.
- Remove the offending control declaration from your handwritten aspx.cs file
- Modify the xhtml (aspx) in some manner that would cause the designer.aspx.cs to be regenerated - i.e. remove the offending control and re-add it.
- Recompile your application.
If this is the source of your problem, you should find that your application will recompile and your intellisense should start working again.