Carrier-side I reckon. Call them to ensure there's nothing at their end responsible. Given what you're already tried, and given your IMEI hasn't gone away, seems a bit curious you'd lose connectivity all of a sudden like that (unless it's a hardware issue).
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?