can't see mp4 files

Win8.1
I can't see mp4 files with file explorer or itunes.
They are there. This is on a hard drive &/or on a sd card on a phone. All other files seem ok

Just out of curiosity here, how do you know they are there if you can't see them?

(There are many valid answers. I'm just curious if you're going to give me one of them, and also, help that will be given depends on it.)

The ones on the phone can be viewed on the phone but not via file explorer in windows when the phone is plugged in. Other files in the same directory are there.
The ones on the hard drive have always been there. The folder structure is there but no mp4 files.

That's rather strange. There are methods to automatically hide files with certain extensions, but you would have had to purposely do so.

I would scan for malware with Malwarebytes.

Also, update any phone software you're using to latest and check that the software is actually compatible with your Windows 7 (I assume, from specs.)

First, open Regedit and delete this key: HKEY_CLASSES_ROOT\.mp4

Then, download this and double click it. (Answer yes if it asks any questions.)

You can test with this to see if you can see them: Download FlashFXP 4.4.3 build 2031 - FileHippo.com

Installed QuickTime & all ok. Thanks all. Weird!