If you are running 6.1.2 as your profile says, it might be Messages was reset during the upgrade process. Go to Settings>Messages to be sure iMessage is enabled. Then drill down one deeper to Settings>Messages>Send & Receive to be sure you have added at least one way for you to send messages and people to reach you.
Do you have a good wifi connection?
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
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8