S3 Screen not turning on. topic

I bought a S3 from someone and he said that the phone needs to be re-flashed. when the phone turns on, it boots up fines. blue light at top, Samsung boot sound, the touch works as u can hear the water effects thing to unlock the phone. he claims he tried the LCD in another S3 motherboard and the LCD works, I don't know if he's telling the truth or not. Can a software cause the screen to not turn on. doesn't turn on at all. I flashed a stock rom with ODIN and did factory reset in recovery mode. i can also go in download mode as well. the phone works well, i used the POWER + HOME buttons to take snapshops on the screen to kinda browse through the phone, everything works, WIFI, bluetooth. it's not picking up signal tho. it's saying "Not registered on network" when i try to make a call, but it recognizes my sim card and stuff. anyways I don't know if he indeed went into the phone's root system files and messed something up or not. please help ! :(