How to fix the No Service error in iPhone 3GS 6.1.3?

So, I have an iPhone 3GS. The previous version of it was iOS 4.2.1, and I'll have to admit that it has been jailbroken.

But now that I've decided that it's time to unjailbreak it, I connected it to the latest version of iTunes and clicked "Restore".So, the iPhone has been restored and updated to the latest iOS that it is capable of: iOS 6.1.3.

Now, everything went well-except for one thing, and that's the annoying "No Service" error. I'm 100% sure that there's a sim card attached to my phone, but even if I've tried taking it out and then putting it back in, the phone can't detect it. I tried activating my phone using the internet, but it says that the activating server wasn't available. I tried the iTunes method, but iTunes said that I can't connect it because it had no sim card.

Yes, I tried searching for a method in the internet, but all of them involved "hacktivating", or activating the phone via hacking materials and potentially jailbreaking it again. I'm DONE with jailbreaking, and I'm never going to go back, but it seems that it's the only way to fix this error.

So, is there any way to fix this error without the means of jailbreaking the phone once again? Thank you very much if you answered.

Added (1). I tried restarting iPhone but the "No Service" problem is still there…

Sometimes your phone needs to be restarted so try holding the lock and home Button until it automatically turns off and turn it back on.