Why did messages quit sending messages?

Yesterday (12/14/17), about 4pm CST, I suddenly could not send messages using Messages on my MacBook Air. I still receive iMessages, and I can send from my iPhone or iPad, but not the MacBook. I have tried logging out of the account and back in, as well as quitting messages completely and restarting. Nothing seems to work.

Anyone else having this problem?

And why can't I open a case on Apple.com about this any more?