Amazon does have a customer service number (1-888-280-4331), but your best course of action to safeguard your account can be done on Amazon’s website.
If you’ve discovered or suspect that your account has been hacked, try logging in immediately and change your password, then remove any sensitive information—including your stored payment methods. Check your order history to ensure that your account hasn’t been used for an unexpected spending spree, and then lodge a report to Amazon.
Always use two-factor authentication, which makes it much more difficult for someone to break into your account even if they know your password.