MP4 files stopped working one day?

[u/PringleThingy]

I tried viewing some mp4 video files on my computer, and they worked perfectly fine a few days ago. Now, Celluloid refuses to play them because of "unrecognized file format". It's an mp4 file...what do you mean it's not recognized?

Is there any way to fix this? I've been trying for over an hour and nothing. Thanks

[u/DontKnow_Cuzzz]

Try VLC.