Apple has admitted that there’s a bug in its new iOS 7 operating system for iPhone and iPad. For iMessage not working problem, you may try:
Step 1: Go to Settings → Messages → Send & Receive
Step 2: You should see your number and an associated Apple ID listed in this list. You can configure this list.
Step 3: You’ll have to try a variety of methods to get iMessage working again. These would include:
- Removing the email ID and using only the Phone number
- selecting either email or iPhone to test if iMessage works with any of these
- select an email AND a phone number. After this, you’ll see another section “Start New Conversion from:” – select your phone number for this.
This works for me. And I find this method in this blog: 3 ways to fix iMessage not working