Before I go on, this is not an everyday problem you come across, but I feel lucky since I resolved it myself. So today I had a little dilemma of my own with Windows Media Player. I had just been playing some videos and unconsciously ran an update which asked me to restart. I was not sure what the update was but upon restart, I tried playing some videos again and all I got was a blank screen with audio. I tried system restore, uninstalling my updates, and upgrading to the latest version of media player with no luck. I almost ran out of ideas. The issue seemed familiar; kind of like playing video on my secondary monitor. All those settings were okay. I then though of my TV tuner having the same symptoms when the overlay setting was on. I thought, what if I messed with the media player settings. I gave it a shot, and it worked. Turning off overlay did the trick.
Go to tools, options, performance, advanced, and turn off overlay settings by un-checking the boxes.
The common, more everyday experience of not being able to play certain video (but most of the time still get audio) is due to codecs. For that, you can go here: http://www.free-codecs.com/