Why isn't my imessage not working all of a sudden?

I've always message people from imessage but from Wi-Fi. Then one day i accidentally turned it off and i tried turning it back on and it always says activation unsuccessful. I even tried reseting my network settings. I have iphone 4s and it does have ios 7. Any suggestions?

Try this solution. It worked for us

http://www.itguyswa.com/imessage-solution-ios7-0-2/