Jose, when replacing the screen on the iPhone 4S, we see it say "no service" almost 90% of the time after replacement. You definitely want to always power your device off before starting the repair. However, when you go to power it on and it says "no service" we find that following these steps help: 1) Connect to a wi-fi network 2) pull the SIM card and reinsert it. That typically works for us. We also will turn airplane mode on and off sometimes too. If that does not work, you may have some hardware malfunctions. A couple of things can happen during a repair; you can easily damage circuits if great care is not taken or we have even seen the EMI shield (the little metal guard that protects the flexes) not be fastened correctly and cause signal loss. Hope this helps!