First of all, you need to change your password if you haven't already. The only way someone could have logged in to your account was to have your password.
Secondly you need to contact Mr Lincoln through the Help & Support tab on the bottom left of this page, or through email at support@wizard101.com
I was playing Wizard 101 with my account when I had a membership. But my cousin deleted my best level 16 Death wizard named Devin Deathsword on purpose. Is there any way I can get it back?
IF you contact our customer support team as soon as possible there's a chance they can recover the character. Send them an email at support@wizard101.com with a polite request to restore the character.