iPhone 3GS network problem

Hi,

I am facing a serious trouble in my iphone. unable to search for network operator. it searches and then shows "NO SERVICE". i took tech help from apple and reinstalled latest iOS software. the phone worked for a day and went kapoot again.

the network bar keep searching for my network operator. i tried replacing the sim with some other operator but it yet inst working.

can someone pls help me

thanks
shef