Stolen phone and thief changed icloud

You will likely be unable to get the account back. If you think your Apple ID has been compromised - Apple Support

A thief would not be able to gain access to your account unless you did not use a passcode on your device.

Apple products are secure. Besides, if someone saw the passcode, the device used would be irrelevant. The weakest points in any system are typically the users. We tend to reuse passwords, make simple passcodes, respond to phishing messages, etc.

You could always try Apple Support, but if the email and password were changed, I'm not sure they would help. You could ask to escalate to Apple Security.