It has been reported that Windows 8.1 update will break previously installed Silverlight Developer Runtime. I can confirm: when I tried to debug Silverlight 5 applications, a message box appeared stating that Silverlight Developer Runtime (aka Silverlight for Developers) is not installed, although trying to get it and installing it again fails, showing that the same version has been previously installed.
To resolve the problem, make sure you download (not just run) Silverlight 5 for Developers to a folder on your computer (i.e. use Save option), then go with a Command Prompt to the folder of your download and run these commands to uninstall and reinstall the runtime:
For a 64-bit machine the commands would be:
Note that you need to wait for the silent uninstall to complete, so wait a few minutes before running the second command (you can check the Windows Installer process in Task Manager to see its progress).
The debugging will then be available again.