what i can do if someone stole my account and change my email address

Contact the Player support for help. :)

Oh my, if they have the old verification system good luck buddy.. You have to answer all their questions which contain some like: Who whas the first champion you played?, Who was the first 3 champions you bought? Give us your first Paysafe's serial number. I hope they have a new way cause this was a nuinsance

Well, I hope you get your help from Riot.