If someone made a video of my song and uploaded it to YouTube: Is that legal? If not, how can I block the video?
That's copyright infringement, and is illegal under various copyright laws.
To take down the content, you'll have to make a claim under the Digital Millennium Copyright Act. The online form for making a takedown notice to YouTube under the DMCA can be found here: https://support.google.com/youtube/answer/2807622?hl=en
Remember, that by submitting a takedown notice, that you are initiating a legal process. Take extra care not to abuse it, and to make sure that all information you submit is accurate.