Many iOS 7 users are complaining of iMessage problems. Some of the issues are translating the message as from another Apple product or non-iOS 7 device. Other issues are messages saying they are sent but only later to show that pesky red exclamation point and letters typed not showing on the screen.
Whatever the problem might be here are a couple troubleshooting measures you can try:
Some of the generic fixes to try out is to restart your device. Another fix is to try to disable iMessage, reset Network Settings and then toggle iMessage back on.
To get to Network Settings go to, Settings > General > Reset > Reset Network Setings.
If those methods do not work, seems like an update will be the fix and will have to wait for it.