Why is my phone saying no service?

The antenna is on the top left hand side of the phone and partially stuck onto the screen. You can easily damage it if you are not careful. It could also be disconnected from the logic board. make sure it's attached to the underside of the logic board. if it still says no service, then replace it. it's a pretty cheap part, only about a couple of bucks. Last resort, logic board could also be damaged (uh oh), but I say try replacing the antenna first.

As for the back glass, just make sure no wires or other screws are blocking the back glass or that the slots on the back glass are not broken or bent.

PChookups think your talking about the iPhone 5 and that's the wifi antenna on the top left of the iPhone 5 not the cellular

The Antenna is connected to the speaker and is a thin black wire make sure this is connected proporly. Also look to the left of were that is connected is the blue coil still attached?

Also make sure that the time and date is correct connecting to wifi usually sets the time and date for you.

Failing that try a restore it can help sumtimes.

Failing that you could try a new antenna

For the back cover make sure it's sitting flat the press down hard if it's still not look at the plastic brackets on rear of back cover see if any are bent.

If so just straighten the plastic back out