Deleted media files

Hello,
I used a computer in our school because my classmate is asking me some files, and my USB appears to have a trojan virus or the shortcut virus but it was fine before not until i plugged it in this USB, I scanned it with the SMADAV which is the only available one and thank God the virus got lost but all my media files are gone, it's all music videos and movies. I thought it was just hidden but when i checked the properties I was so shocked that there is a 28 GB space which is supposed to be only 1 gb space, Why is it that only my video files got lost and not my documents?
I have no back up for those files, please help me!