Can you help me to find my IPhone 6 by the imei

Solution
https://support.apple.com/en-ca/HT201472

"What if your iOS device is off or offline?
If your missing device is off or offline, you can still put it in Lost Mode, lock it, or remotely erase it. The next time your device is online, these actions will take effect. If you remove the device from your account while it's offline, any pending actions for the device will be cancelled."
Only your Service Provider can help you with this.

You're lucky it's offline as you put it - that's better than running up a huge bill for which you are responsible unless you notify the company.
 
https://support.apple.com/en-ca/HT201472

"What if your iOS device is off or offline?
If your missing device is off or offline, you can still put it in Lost Mode, lock it, or remotely erase it. The next time your device is online, these actions will take effect. If you remove the device from your account while it's offline, any pending actions for the device will be cancelled."
 
Solution
you should also file a police report, its illegal to steal someones personal device, because they might be smart and not put your phone back online