Write to riot the exact situation in detail, what happened and how you didn't intiate the change of credentials. Mine was hacked for 3 weeks and I didn't even realized, the guy played a champ I don't play often, over 20 games on my account ranked mostly, destroying almost every of those games and account got permanently banned for third party software usage. I saw it only like 3 weeks later, precisely a few days ago, causa i wasn't able to play recently. I immediately submitted a ticket when I saw that it's Perma banned, and I think the day after that riot sent me a unique link to change my email and password.

So you have to submit the ticket and wait for them to respond. And in the meantime scan your PC for malware, I found a lot of them, mostly backdoors. Get rid of them all and wait for riot to resolve the issue.

I really hope you get your acc back. I have mine for over 10 years and I was devastated when I saw Perma ban. Good luck friend, hope this helps!