So Let's get Started
1. Restoring by Flashing Kernel's
While flashing Rom's your IMEI number goes , so just after that flash any Custom Kernel and IMEI number gets back.( it works in 10/7)
2. Going back to Stock ROM via ODIN
Yes, by flashing Stock Rom with wipes your IMEI number gets back.
3.By HEX EDITOR Method
Here i have made a special Video for you Guys who does't understand how to use Hex WorkShop so just watch these video .
Note:By following Above listed Methods your IMEI number get restore , but if someone's not getting than i don't have any other solution or you have to give it to Service Center
I hope these Thread you will found Help full