network not registered for galaxy mega I9200

hi there
i had a problem with my mega GT-I9200
yesterday morning last call and it was everything working okay but after a 30 minuets i had a sign that network not available
first i am in turkey right now and i don't know if there is somebody can help for this problem
there is a roles in turkey that if you get a cell phone from out of turkey it will work only 60 days and the company will close your network if you didn't pay for the TAX office in turkey ... and i am not sure about my cell phone
there is some shops here just can change IMEI number and maybe fix network for the cellphone and it will work again
BUT i really did it i change the IMEI after i root my cellphone and still have this problem only with Turkish sim cards i use my Lebanon sim card and it is working
so i don't know exactly should i change modem file
or use other ROM
or what i can to do ???
please i need some body can help for that

i am working on it since yesterday morning but still the same
........