This is part of my Project.sln file:
Microsoft Visual Studio Solution File, Format Version 10<strong>.</strong>00
# Visual Studio 2008
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MyApp", "MyApp\MyApp.csproj", "{58F9C679-A58F-4942-AE06-C4B03E398256}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MyApp.Core", "MyApp.Core\MyApp.Core.csproj", "{DD7B354E-20F2-405C-95BE-67E6E94EBB53}"
EndProject
You need to update the fields marked in red above. In this case the projects were housed in:
F:\DotNet\SomeProject\MyApp.sln
F:\DotNet\SomeProject\MyApp\MyApp.csproj
F:\DotNet\SomeProject\MyApp.Core\MyApp.Core.csproj
See how the second part is the relative file path from the .sln? That needs to be changed