Chosen Solution
I replaced the screen on my iphone 3gs a few days ago. Reassembled and now I have No service and no sound. Everything else works fine, including wi-fi. I tried resetting the network setting and I will get 1 bar of signal for all of 1 minute before it goes back to no service. I also checked the two metal tips which are both touching the metal case and there doesnt seem to be any damage on the board by the do not remove sticker. All my connections are good also. I am at a loss. The phone is still under warrenty but while I had it tore down I decided to paint the shell so I would assume that voids the warrenty. Any ideas or is it a lost cause? Update Sanded the paint off and still nothing. Reset network settings and same 1 bar then no service. It wont hold a date and time either, I set the date and time restart and it is back to dec 31 1969. I am at a loss and at this point am seriously regretting trying to repair the screen myself because now I have no phone at all. :( ==Update== Phone is now working again. The really tiny inductor by connection 7 was missing. Small amount of solder to reconnect to two contact points and I have full service.
No Service usually means that the iPhone in not getting good reception. It could be Carrier Problem, SIM Card could be faulty, or Antenna issues as you have already checked. Try another SIM Card from someone who gets good reception, or get a new SIM Card from Carrier. Or clean metal contacts on your SIM Card with some Isopropyll Alcohol. Try Resetting the Network Settings again in the Settings App. Last step would be to open iPhone again and make sure the two copper pins are making strong contact to metal Bezel. Make sure that Flex Cable #7 is securely fastened correctly. Make sure the Antenna Metal Loop is on top of the Bezel when screwed down. If you did not remove the Dock then this should not be an issue. Maybe the new paint is interfering with the reception, that would be bad. With iPhone 3GS make sure iPhone is OFF before opening and during all repairs.
