Hey there Cindersue,
It sounds like you Messages app is becoming unresponsive when you try to edit your text even after having restarted it a couple of times. I would next quit all the running apps on the phone:
iOS: Force an app to close
http://support.apple.com/kb/ht5137
Double-click the Home button.
Swipe left or right until you have located the app you wish to close.
Swipe the app up to close it.
When you have done that restart the phone and test the Messages app again:
iOS: Turning off and on (restarting) and resetting
http://support.apple.com/kb/ht1430
If the issue persists, does it happen with more than one contact?
Thank you for using Apple Support Communities.
Cheers,
Sterling