A big wtf here....I've been watching a course of MP4 videos with Windows Media Player for the last couple of weeks. Watched half of one this morning. Went to play it again just now to watch the other half, and I'm getting a "Windows Media Player cannot play the file" error message. What on earth could have possibly happened since this morning and now to suddenly make MP4s unplayable in WMP? I hate baffling crap like this. 

Well, as quickly as I posted this, I solved it. The audio output device associated with WMP had inexplicably changed itself to "default audio device." I really wish I could reach into my monitor and slap Windows sometimes.