Video download problem(RESOLVED)

This is my first time here so a big HELLO to everyone. I'm not too literate when it comes to computers so bear with me please. Something happened, probably inflicted by my teenage son, when I download videos from blogging sites. All of a sudden when I try to download the video plays for a few seconds and disappears from the screen and the sound continues. I can however watch videos from sites such as the BBC. Where do I start?? - Ozymandias.