You didn't post the error so I'm not sure how we'll help.
In general if you just drag the .aspx file within Solution Explorer from one project to another (or across VS instances) then it'll copy the subfiles. More importantly though it needs to set up the subtype
on the child projects otherwise you end up with extra designers and things don't work. Open your project file in a text editor. The extra files need to be added as subtypes of the main file. If you drag and drop in Solution Explorer it handles this automatically. If you do it by hand then it might not.
<Content Include="Index.aspx" />
<Compile Include="Index.aspx.cs">
<DependentUpon>Index.aspx</DependentUpon>
<SubType>ASPXCodeBehind</SubType>
</Compile>
<Compile Include="Index.aspx.designer.cs">
<DependentUpon>Index.aspx</DependentUpon>
</Compile>
However if you simply copied the files from one folder to another using file explorer then the files aren't even in your project. You need to add existing item to get them there.
After the files are properly added to the project the next hurdle is probably the namespace. You need to edit each of the files and change the namespace references to whatever your new project is using. In the .cs files that will be the namespace blah
line at the top of the file. You might also need to adjust your using
statements. For the aspx file it'll have an attribute that references the codebehind file. That reference will be a full type name including namespace. Change it to match whatever your .cs file is now using.
At that point the designer should successfully open and be hooked up. However you may still have compiler errors if your files are referencing things from the old project that you didn't bring across yet. You'll need to manually fix them up.