Nexus S I9020T does not work on mobile network state as "connected". It really works at home and office network on wireless acess points, but not at mobile network operator. This service is provided by the operator and works well on other cell phone, but not at this moment at Nexus S. It use to work well. Can i fix it by by my self or I have to deleaver it to Samsung costumer service? I do appreciate any help.
If you're using Sprint or Verizon (Or any other CDMA Network) and bought the phone used, then there is a chance that the device has a bad ESN number. It means that the phone is "blacklisted" and the network will not allow it to connect, because it has been reported as stolen, or it's original owner had canceled the network plan without paying the cancellation fee. Here is a thread that explains the full meaning of this. If the phone doesn't have a bad ESN, then do as rab777hp said, and check your carrier settings / obtain a new SIM card.