Sometimes it happens if you either:
- Clean solution/build or,
- Rebuild solution/build.
If it 'suddenly' happens after such, and your code has build-time errors then try fixing those errors first.
What happens is that as your solution is built, DLL files are created and stored in the projects bin folder. If there is an error in your code during build-time, the DLL files aren't created properly which brings up an error.
A 'quick fix' would be to fix all your errors or comment them out (if they wont affect other web pages.) then rebuild project/solution
If this doesn't work then try changing: CodeBehind="blahblahblah.aspx.cs"
to: CodeFile="blahblahblah.aspx.cs"
Note: Change "blahblahblah" to the pages real name.