I have imported .mp4 files before, but now I can't.

I am fairly new with this program, and I can recall playing around with some of the imports. I remember importing a .mp4 video I have saved into my files, but now when I follow those same steps, it loads, but nothing happens. I have heard of people importing these files, and receiving nothing but black frames, but my result is nothing. No changes. It would be highly appreciated if someone could assist me with my current situation.