Why wont my mp3 play videos?

Well i had alot of space left afetr having music it on it. I downloaded a video about 21 minutes long, and my mp3 does have video on it. I had totaly enough space and idk why it wont play. On my computer it says the video is on there, but when i unplug it and go to videos from my mp3 its not there. (rca Opal m4304)