Default Mobile network not available

On my Samsung Galaxy S3 (d2mtr) it keeps giving me a "Mobile network not available" error whenever I try and make a call. I can go on the web and social networking apps; download apps. The only thing I can not do is make calls. I looked under settings and saw that "Network Operators" is not available to enter. Any ideas on how to fix this??

EDIT: My IMEI number does in fact show up. Idk what is wrong with this.

EDIT: NEW PROBLEM. I was reading a tutorial about flashing back to stock firmware and it said to downgrade my bootloader. I used the recovery flash method and now I have a black screen. Can't do anything. Turn off, turn on. Just stays black with no response. Someone help fast Please