Contact AOL and explain that exactly what you have said here. Ask them to freeze the account until you can prove you are the rightful owner.
Also, use a password manager and never use the same password twice on different systems. On very lar attack hole is people, using the same email and password on multiple systems. I also recommend a backup email account, there are many free ones, where you can list on many systems the second email.
One of your largest concerns at this time might be this person using your email to change passwords of yours on other systems. Once they have control,of your email, they can make password change requests to your own email, and change the passwords on other systems as well.