Silverlight project becomes unloadable

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Silverlight

I have a Silverlight 5 project. It run well till yesterday. This morning, my IE continuous popped out a message to ask me update something for Silverlight. After I did, my Silverlight project becomes unloadable. When I re-open the .sln, the message shows:

Your need to to install the latest Silverlight Developer runtime before opening Silverlight prohect xxx.
Download the runtime at http://go.microsoft/fwlink/?LinkId=146060

After I clicked the OK button, nothing happens. Besides, the website: can't reached anymore.
Then, I referred to How to convert silverlight 4 application to silverlight 5 - Microsoft Dynamics CRM Community[^] to open the .csproj file. I observed that a blue waved line is underlined the TargetFrameworkIdentifier. The message shows that

The element ‘PropertyGroup’ in namespace ‘http://schemas.microsoft.com/developer/msbuild/2003’ has invalid child element ‘TargetFrameworkIdentifier’ in namespace ‘http://schemas.microsoft.com/developer/msbuild/2003’.  List of possible elements expected: ….

Do you know how to solve this problem? Thanks.

What I have tried:

Silverlight Project becomes unloadable