Sending and receiving messages

Most probably you need to contact Ring Central support and something needs to be done on their back-end to enable SMS again.

Why was it disabled? no idea.
The best thing you can do is go to the admin portal > More > Audit Trail and filter for your extension and search if any changes were made to your extension. Likely you won't find anything there because this issue sounds like something that only RC can solve themselves.

All of this is assuming that you actually have a direct number, and the message that gets displayed to you is wrong. I hope you know how to check this already.