Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: FaceTime is 'Unable to verify email because it is in use'
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage
How to Set Up & Use iMessage on iPhone, iPad, & iPod touch with iOS
Set Up Alert Sounds
Extra FaceTime IDs
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Fix Can’t Sign Into FaceTime or iMessage iOS 7
FaceTime, Game Center, Messages: Troubleshooting sign in issues
Unable to use FaceTime and iMessage with my apple ID
How to Block Someone on FaceTime
My Facetime Doesn't Ring
Send an iMessage as a Text Message Instead with a Quick Tap & Hold
To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.
That is really a broad statement/question?
If you can't send messages go to Settings>Messages>Show Subject Field>Off.
Mesaages only works with other iOS devices or Macs running Mountain Lion or Mavericks and the person that you want to message with must be setup and registered to use messages on their own device before you can text with them.
If none of that was applicable, please ... By all means ... Actually describe what is going on. Don't keep us guessing with statements like "iMessage not Working"?
It worked up until last night. One message to my husband went through and then all subsequent failed. I didn't even know what imessage was until it failed. I have restarted my phone (home & lock held down) and it didn't work. I have even turned it off, reset the network functions and turned it back on and no dice. This needs to be addressed!
iOS 7 iMessage “Waiting for Activation/Activation unsuccessful/Not Working” Fix
After not finding a solution this fix I found by myself worked for me. Please
note that it may not work for everyone. (Found on an iPhone 5)
Step 1: Go to Settings > General > Reset > Reset All Settings
Step 2: Your iPhone should reboot so hold on for one second.
Step 3: Go to Settings > Messages
Step 4: Turn on iMessage and then double click the home button
Step 5: Turn off Settings by dragging up the application window with your finger.
When you go back to settings it should work.