stolen device

If it was signed out of iCloud (to disable activation lock and find my iPhone) then someone must know your AppleID password as that is required to do that. So first, Secure your AppleID - If you think your Apple ID has been compromised - Apple Support

If it is still listed in your iCloud account but shows offline, then all they’ve done is erased it or turned it off.

Then report the theft to your carrier and local police, and insurance company if you have insurance on it. Other than that there is nothing else you can do if it has been signed out of your iCloud account.

The only steps you can perform are listed in this support guide:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Regards

Giulio

Ask your service provider to lock your device.