What messaging application are you currently using? Have you attempted to clear the cache and data for the application yet?
Settings > Apps > All Tab > *Messaging* (or name of app you are using)
Additionally, is you are seeing a message in the text box stating "Sending Disabled", please verify whether Hangouts or Mesaging is your default text messaging application. You can check by going to
Settings > Wireless & Networks - More... > Default SMS app
If Hangouts is selected, you can either use it for both Hangout chat and text messaging or change it to your preferred messaging application.
Thanks to this forum I found the right answer (reset default messaging app).
But this "default" keeps getting CONSTANTLY reset to another messaging app on my
phone. I don't ask for it, it just reassigns itself. Then it comes up "messaging disabled".
The software is BUGGYYYYYYYYYYYYYYY
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