Someone hacked my account

I have not played LoL in quite a few months. I just received an email that said my account was perm banned due to use of 3rd party. How can I recover my account I know someone can look at it and see it was not my IP address. I have never given anyone access to my account, I have never tried to use a 3rd party app. Sh*t I barley ever made it out of bronze, but I spent alot of money on skins(which dont actually make you better). I didnt even have the client updated. Please someone help!!!!!!!!!