How do I fix failed activation and undelivered Messages on iPhone?

Every time I try to send a message on IMessages it doesn't deliver. It used to say I was signed out but that test doesn't appear anymore. When I check the app in settings and try to activate my send and receive it, it tells me it was unsuccessful. Another thing that happens is every time I try to click my number in the activation it tells me there was an error during the process. Can someone please help me?

[Re-Titled by Moderator]

Original Title: Problem with IMessages