I can not send messages on iMessage - can receive

Have you tried power down and powering back on or reseting the unit?

to reset hold the power and home button at the same time for 10 or so seconds untill it resets and you see the apple screen.

http://support.apple.com/kb/ht1430

have you tried an entirly new message or another contact with same resault?

or deleteing the old message and starting a new one?

your welcome

I have tried that. I can send messages to others but not this one person!!!