Regenerate aspx.designer.cs file
Seems like the connection between your asp and asp.designer.cs file has stopped working? Here's a two step guide how to regenerate it.
Within the Visual Studio:
1) Remove your aspx.designer.cs file
2) Right click on your aspx file and select "Convert to Web Application"
This should add the aspx.designer.cs back and up to date.
If you get an error saying:
"Generation of designer file failed: The method or operation is not implemented."
Try close Visual Studio and then reopen your project and do step number two again.
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