Have you tried using the troubleshooting steps on the main page of this help forum?
While I have an IPhone, I have found that sometimes enabling airplane mode for a couple mins then disabling airplane mode fixes some quirks I have ran into.
Also, I have had a couple times where iMessage gives an alert that the message did not go through, but the party texts back and it is obvious they did get it. Saying this to say, are you confirming with the other party they didn't get the message or are you going off a delivery/read receipt on the messaging platform?
Based on some other response, it appears this may be a more widespread issue. I assumed this was an ongoing problem and not something that just started. I would suggest either reaching out to chat directly or waiting a short period of time to see if it resolves...