VLC won't play .avi or .mp4 files?

So, I was trying to play some video that my cousin sent me that was in the .avi file – I always use VLC for my videos and they have always worked out well in the past, except for today.

I also tried Quicktime and Realplayer, but they either say that they do not support the format (Quicktime, which is understandable), or that they do not recognize the file.

What’s up? How can I fix this?