WORD on that ! I have done the same: googled everything & tried every combination of the suggested fixes (airplane mode, auto-time, location services, reboot, log in/out) and I CANNOT get iMessage to activate again. Regular SMS is working fine.
There here have been some reports of a *time-dependence* in recent incidences of this problem: on one of the iPad boards, there were reports that only Apple IDs created after March 2015 were having difficulties. My time-dependence issue us different : I'vet been using iMessage for ages on my iPhone 3GS, then 2-3 weeks ago, it abruptly bumped me out of iMessage and has since refused to reconnect. (I only found out because I was getting iMessages on my iPad - same account - but noticed my iPhone was not chirping at the same time.) I changed absolutely nothing on the iPhone anywhere near that time.
My immediate suspicion was that Apple had dropped iMessage support for iOS 6 -- I can't upgrade the OS any further because it's an "antique" 3GS. However in all of my googling, I have not encountered anything to support this suspicion. Everyone recently encountering this problem seems to be on iOS 7 or 8 ... but for me, iOS 8 on my iPad has no trouble with iMessage. Since my sister back home in Canada and students on summer break in Asia use iMessage exclusively to save SMS long-distance charges, while everyone on non-Apple phones uses only regular SMS, I must now carry around iPhone AND iPad everywhere.
I feel your pain, Hannaabe. 😠The post by horsetoothres at the bottom of https://discussions.apple.com/thread/6789149?start=405&tstart=0 provides some hope that the problem is known to Apple. Cheers.