This is a crucial problem for you to handle. Well, for any device it is must to have its documents. Somehow and somewhere it would be helping you. But you have lost all the details. Even then, it could be possible for you to fetch the IMEI number.
Method – 1
If you bought your mobile through an offline retail shop, you may go there and enquire about your registration for the device. They may have a copy of your registration bill. In that registration you may have given the contact number and mail id, through that you can get your details easily. In another scenario, if you bought your mobile through online shopping, you may receive an invoice copy for your purchase in mail. You need to download that invoice from your mail. In that invoice you can get clearly the IMEI number of your device.
Method – 2
Another way to find your device IMEI number is through your Google account and that would be attached through your device even if your device is reset. But the information of your device has been stored in the Google Dashboard. Simply login to your Google dashboard and there you could find Android option. There you could view the listed details of the device. From there you could get your IMEI number. But if you haven't linked your device with your Google account, it would be difficult for you to get the details.