If you don't know IMEI and haven't turned on localization on Google, you can't get it back the easy way. Normal procedure is to report it to the police and ask telco to disable the phone (you need proof of purchase). The phone will then get useless (depending on country and operator, some operators and countries share informations about stolen phones, some don't). Sometimes telco can also track the phone (if you bring police report and proof of puchase) and localize it.