After render a project using Vegas Movie Studio 12 the Windows Media Player stop working. When I open a video WMP looks like it's trying to play the video except nothing happens. The only way to fix the problem is reboot the computer.
I too have had this problem. I'm not sure why it does it, but the re-booting of the computer will fix it. I wonder if it is a glitch in WMP. Sorry I can't help you.
MM