If you think your account has been hacked or taken over, there are several actions you may be able to take via the website or the app to secure your account.
If
security@mail.instagram mailed you a message that your email address has changed, you can undo this change by choosing to protect my account in that message. If the other information has also changed (for example, your password) and you can't change your email address back, ask Instagram for a login link or security code