Several users have reported iMessage issues after updating to iOS 7. iOS (iPhone, iPad or iPod touch) users who can’t send or receive messages after the update indicated that they were not having any issues with iMessage before this upgrade.
If you’re experiencing the issue, here’s how you can fix:
1.Tap Settings > Messages and disable Messages by toggling it off.
2.Reset network settings by going to Settings > General > Reset > Reset Network Settings
3.Tap Settings > Messages and re-enable Messages.
If above suggestion does not fix your problem try:
1.Reset your device by pressing and holding the on/off (sleep/wake) and the home buttons together for about 10 seconds until you see the Apple logo.
You may also want to change your DNS settings to fix this problem.
More: Messages (and iMessage) not working? How to fix your problems