Phone working then No Service

Make sure you are on the latest update and connect to Wifi to make sure you don't have any Carrier updates available. Also try the following steps from the Apple Support Article (https://support.apple.com/en-us/HT201415).

• Check for a Carrier Settings update
• Take out the SIM card and put it back in
• Reset your Network Settings.

A couple of things to check would be to make sure your SIM card is active. If it is, then disregard that. Check to make sure your SIM reader is working... If you remove the SIM card it should switch and say NO SIM. If this happens, leave the SIM card out for at least 30 seconds and then put it back in. If it doesn't work after doing this... You may have another issue. I have seen where phones that are slightly bent causes the phone to not be able to find service. I do not recommend bending your phone back into position with the screen still on, you will risk breaking the screen. If none of this applies to your situation, check your IMEI against the national database and make sure your IMEI is not "blocked" which makes it so it cannot be used on any carrier's service, even it's original. If this is the case, and you're the original owner of the phone, a quick call to your carrier's customer support line should solve this issue.

For what it is worth, and I know this is over a year old, but I had the same issue, and it turned out to be a bad sim card. I tried everything short of opening the phone (that was soon approaching though). On a whim I tried another sim card and viola...I tested to make sure it worked by making a call. When I switched the sim card back the problem came back.