Hacked and Banned pls help :/

Hey I need some help trying to resolve an issue. A month ago I was playing and after every game I'd get logged out of my account with a notification saying "This account has been logged from a new computer" or something along those lines and then I found that whatever had gotten into my account had deleted all of my friends. So I of course attempted to make the correct provisions and change my password, nothing out of the ordinary happened until I logged in yesterday after being gone for training at Ft. Custer when I tried to log-in and it said I was perma-banned. So I went to check my email and found whatever that had gotten into my account a month ago got in again and was using a "Malicious 3rd Party program" under my name and got it banned. Now the question is, How did this happen? I scanned for shit on my computer and nothing popped up and there were no key loggers as far as I know? Is there any chance I can get my account back? I have screenshots my friend took of my match history and all of which were champs I don't play. If anyone could help that would be amazing