Sony Ericsson t200 " No Network "

Hello

I unlocked a T200 (locked to AT&T) with a Sony Ericsson clip (AERICLIP III)by reading the codes. Now that I have unlocked the phone and powered on with a different sim card it searched for the network for a while but now it says no network. I relocked the phone with Sony Div 7.1 but phone still saying "no network" it is no longer searching at all.
Can someone tell me how to fix this problem. I tried selceting "rebuild sp area " function but not working. Do I flash. I dont know which file to use.