enter code hereThe error you are getting means that there is no file called default.aspx.cs in the same directory as your default.aspx file.
Please make sure that this file exists.
You may have deleted it, or moved it to a different location.
Your IIS is trying to link the default.aspx.cs, which is the code file, to the default.aspx file, which is your HTML file, which uses the code.
You can see the link between the files at the top level declarations in your ASPX file:
"Codefile = default.aspx.cs"
If you cannot file the default.aspx.cs file, I suggest you re-create the ASPX file using your development editor. This will create the default.aspx.cs file automatically under your ASPX file.
From MSDN:
The code-behind page model for Web Forms allows you to keep the markup
in one file—the .aspx file—and the programming code in another file.
The name of the code file varies according to what programming
language you are using. For example, if you are working with a page
named SamplePage, the markup is in the file SamplePage.aspx and the
code is in a file named SamplePage.aspx.vb (for Visual Basic) and
SamplePage.aspx.cs (for C#).
Hope this helps.