My iphone can not work on another carrier's network. What can I do?

A year ago, I buy an iphone and use in usual.

Buy after I upgrade to IOS5,it doesn't work any longer.

Someone say iPhones designed to work on AT&T's network will not work on another carrier's network.

Amazing! Before I upgrade to IOS5,it works on CMCC,and it can work any other carrier's network.

What can I do?