EFS backup ? Restore it. How ? Search. Many, many, threads on the subject here.
No EFS backup ? Get some stock roms for your country/carrier from Samfirmware & start flashing. You may get lucky & the IMEI loss may be temporary. If that doesn't bring it back, many local mobile repair shops have equipment that can re-write the IMEI back to the phone fairly cheaply.
And finally, if you want meaningful help rather than generalisations, you're going to have to provide a lot more information (this goes for future threads you make as well). Without rom, kernel, Android version, rooted/not rooted, exactly what you've been doing to the phone lately you limit your chances of getting help beyond what I've offered. We shouldn't have to ask you this stuff/engage in 'pulling teeth' to extract it from you.
Last edited by MistahBungle; 30th April 2014 at 05:54 PM.