Another thread about to be locked or deleted. IMEI discussions are not permitted on this site due to abuse.
You can find the information you need from other sites via Google. Reflashing your modem, letting the phone catch a signal and a little time /patience does resolve the problem for many. Once you get back to normal, make sure that you back up your entire EFS /NV in case you run into any future problems.
As others pointed out, do not try to downgrade a 4.3 bootloader. 4.3 was a major update and blew a qfuse in our phones. Meaning that trying to revert it will likely brick your phone. If your IMEI is actually corrupt don't panic. You just need to spend an hour or two of quality time with Google. The information you need isn't that hard to find, it's just buried in an avalanche of help me posts.