Musical Routines w/ Video, Not Playing Video

[TastyHamSandwich]

Recently I completed a project for a client that involved a number of musical routines tied to .MP4 files, using video I had made for them. Everything worked perfectly on my end for months on end, up until today when I get a tech call from the client saying when he plays two particular video files, he gets audio but no video player pop-up as is normal. He still gets video on several other video routines I made for him, just a couple of them.

I went to test these two routines on my machine, and suddenly I do not get video on them either. The files already exist on a third machine, so I go to check those copies - same thing. I have no idea when this started or what happened, but it seems these two files will play audio, but not launch the video component. I can play these MP4 files independently, and the video output works just fine through Windows Media Player.

Has anyone else ever experienced this? Does anyone know where I might start with trying to figure out what has happened? It *MAY* have something to do with having set options under the Sequence Editor, under Edit > Preferences > Video Preferences, setting it to Fullscreen Playback, and in the case of my client, setting the playback monitor to another device. I'm currently at a total loss right now, and could use any and all help that anyone can offer. Thank you in advance!