Go on the your carrier’s website or call their customer service department for assistance. If you have a late model phone you might have a phone finfing feature. They will ask you information and tell you how to proceed. If you have insurance, they will replace the phone if you haven’t used it in the recent past.
Gmail may help you !
But if someone has stolen and flashed your phone u r not likely to find it this way.
Now go to police and file and FIR give them your imei no. they will share them with telecom companies if someone inserts other sim on your phone and use it they will get caught.