My iMessage won't send I have an iPad 2 and have used iMessages many times. For some reason now I can't message. The send button is not active.  I'm trying to message a friend who has an iPhone and who I have messaged many times in the past.  Any clue as to what to do?

I have force closed the app, powered off, checked the contact number, all to no avail.  WiFi is active. Have gone thru all steps listed as possible fixes except deleting and reinstalling.  I can't find iMessage in the App store so I don't want to delete because I don't know how to reinstall.
Turn iMessage off, hard reset your iDevice and then turn iMessage back on again and it will start working again! I've had this issue on more than one device, most recently after updating to iOS 6.