This problem can be related to different things and they all have solutions. There is not a solution that fits for all the phones. Solution is depending on your problem.
1.Disconnected Service By The Carrier
First thing first, calling the carrier and checking on your account would be a good start to fix this problem. Carriers disconnect services for many reasons so it is always good to check with them to see if they stopped the service for some basic reason. Because iPhones say No Service if your service has canceled.
2. The most common problem is the software update. A lot of people’s iPhones started having No Service problem after iOS updates. There is also a rumor about Apple does it on purpose to sell their newer products but let’s focus on the solution now.
After software releases some problems may occur on some models and new iOS update generally comes with bug and software related problems fixes like this so It is always better to keep the software up to date.How to check the Software updates?
Software update options are available on Settings->General->Software Update.
3.Resetting Network Settings
This method is always useful to fix problems related to wifi and cellular. By resetting the network setting you will loose the Wi-Fi passwords that has been used. You will have to re-enter the passwords in order to use the Wi-Fi.This is how you reset the network settings:Settings->General->Reset->Reset Network Settings.And follow the instructions.
4.Back Up and Restore Your iPhone!!
Before you restore your iPhone it is highly suggested to back up your phone via iCloud or iTunes to avoid data losses. We are not responsible for any losses or problems after using this method. If you don’t remember your iCloud password you phone will be locked and there will be no access to the iPhone without using the Apple ID that has been used to activate to phone at the very first time. On the other hand when you are experiencing No Service issue there is not much to loose anyway.
5.Hardware Repair Options.
Apple Store doesn’t offer any kind of repair options if it’s hardware related. When you buy a device it comes with a limited warranty for 1 year and you can also purchase their Apple Care coverage. For this problem Fix 4 Less San Diego offers a hardware repair option. It comes with a warranty for 90 days and repair takes about a day.