iPhone Stolen and ICloud hacked/changed

There are a few things you can do in that case:

• Mark it as lost
• Remotely erase it
• Contact Apple Care via the Apple Support

Contact Apple Support - Apple Support

...

If you use Stolen Device Protection for iPhone, Face ID or Touch ID is required to turn off Lost Mode — so even if a person has your iPhone and knows your passcode, they'll still be locked out of your device.

1. Go to iCloud.com/find

• You don't need a verification code to sign in, so you can sign in and mark your device as lost even if your trusted device was stolen.

2 . Under All Devices, select your iPhone or iPad.

3 . Choose Mark as Lost, then follow the onscreen instructions.

...

After you mark your device as lost

• Don't include your contact information if your device was stolen. While you might see an option to enter a phone number or message about how to contact you, this information is helpful if you've lost your device and want to make it easier for someone who finds your device to get it back to you. If your device was stolen, the thief might use your contact information for social engineering schemes.
• Don't remove the device from Find My. Removing the device from your Find My list removes Activation Lock, which will make it easier for the thief to erase and resell your device.
• Mark as Lost, even with Stolen Device Protection. Stolen Device Protection's additional safeguards last only for a period of time. The best way to protect your iPhone and Apple Account is to quickly mark the stolen device as lost.

...

• Report it to local law enforcement. They might request the serial number of your iPhone or iPad, which you can find even if you don't have the device.
• Contact your wireless carrier to report your stolen iPhone. You should ask them to suspend your account and file a claim if your iPhone is covered under your wireless carrier plan.

...

If you can't recover your stolen device, remotely erase it and review your Apple Account information. If your iPhone is covered by AppleCare+ with Theft and Loss, you can also file a claim for a replacement.

...

Make sure that you've tried everything else to find it — erasing your device can't be undone.

You can remotely erase your device using the Find My app on another device or at iCloud.com/find.

• If your device has iOS 15 or later, or iPadOS 15 or later, you can still use Find My or iCloud.com/find to locate it after you erase it.
• Don't remove the device from Find My, even if you remotely erase it. Removing the device from your Find My list removes Activation Lock, which will make it easier for the thief to resell your device.

If your device is offline, the remote erase process begins the next time it's online.

Remotely erase your iPhone or iPad on iCloud.com/find

Remotely erase a device in the Find My app on iPhone

...

copied from: If your iPhone or iPad was stolen - Apple Support