How can I fix my MP4? I tried to format it but it didn't work?

How can I fix my MP4? I tried to format it but it didn't work.?
how can I fix my MP4, I tried to format it but it did not work?
how can I fix my MP4?
I connected it to my computer to add songs then what hapens is all my files were gone.
to check if my files were really gone, I look into my MP4 and I saw my files was still there.
Then I check my MP4 in the computer again, my files were really gone.
I tried to transfer some songs from my computer to my MP4 but it didn't work.
I tried to format it because thats what I do when I'm having troubles with my MP4 but it did not work?
When I disconnected my MP4 to the computer, all the files in it were gone.
Help, what should I do?
I really need help, please....
(I also ask this question in the MP3 categories)