I got banned for something I did not do

last time I had played other than trying to get online today was on April 17, 2017. I got online today and before getting a notification for my ban, I had realized that 12 000 of my ip was used on things I hadn't purchased personally, and I also had 16 games played on my account. These games that were played were dated on April 20, and April 21, but I had not ever logged on, on those days. I believe someone had somehow got a hand on my account without my permission. I had not ever shared any information with another player on League of Legends, and have never in my life used a 3rd party program. As you can also see in my match history, I had never played Xayah before, and that the person playing on my account had a much higher k/d/a ration than my average. He had also changed all my masteries and runes along with using the ip I had to buy new ones. While playing on my account, he unfairly moved me up by 2 divisions, which I can see can make the game a lot worse and ruin the game experience for other players facing the person on my account. I sincerely hope that we can do something to solve this mix-up and hope that you understand I was not behind this problem.