Julia, I went to youtube to see if I could report this scammer. You can report a channel, but you have to choose from specific reasons for reporting. There is no "someone hacked this account" choice or any other one that makes sense. Then I checked to see if you can report a video. You can, so for the reason, I chose "spam or misleading". Then you can type in a text box for further clarification. I typed in that someone hijacked your account. It said that these reports are monitored 24 hours a day. Perhaps you can ask others to report that video also? Under the Bill Gates video, there's a flag icon to the right of "like, dislike, share, save". Click the flag to report.