Fixing error Silverlight.Csharp.Targets was not found

If you are facing build errors where some (likely projects that rely on Silverlight) of your projects are not being loaded into the solution, here's what worked for me (I used Visual Studio 2015, but the same should work for 2010):

(The solution is based on what was proposed here).

1. Go do Programs and Features, select your Visual Studio installation, and run a Modify (if not present, you may be forced to go with Repair). Check the checkbox that mentions Silverlight
2. Afterwards, when reopening Visual Studio to see if the project(s) is/are correctly loaded - you will be prompted to install Silverlight Developer - the pop-up will include the download link as well. Download, install, and you are (hopefully) done.