iMessage activation unsuccessful

does anyone know how to fix their i phone when it says "activation unsuccessful" when trying to turn their iMessage on? because out of no where my i message stoped working. my face time started calling using my email which i don't like and my iMessage sent as green texts. so i rest my phone and its still the same and now it wont let me log onto i message of face time using my apple ID :/ and its been like this for a while i don't know if this made sense but i know A LOT of people have gone through this same issue:/