Email accounts are especially important because they are often used to reset passwords for other services.
Change your email password immediately from a device you trust. Use a new password that is not used elsewhere.
Sign out other sessions and remove unfamiliar devices or connected applications.
Check your recovery email address, phone number, authentication methods, forwarding rules, filters, and automatic replies for changes you did not make.
Review sent messages and account activity for actions you do not recognise.
Enable multi-factor authentication if available.
After securing the email account, review other important accounts that use that email address for password recovery or security notifications.