iPhone 5 my iMessage doesn't work
If you have reinstalled the operating system of Apple just decides to make another change I have not quite figured out yet, I often find myself needing to go in and reenable the iMessage and FaceTime feature. It is fairly straight forward

Go to Settings>Messages and turn off iMessage.
Then go to FaceTime and turn off FaceTime.
Restart your device.
Turn iMessage and FaceTime back on.