After it was experiencing issues all day (at least here in the LA area), it seems Google Hangouts is currently down for a good portions of folks living in the US and probably other parts of the world. Downdetector.com is showing issues all around the globe, with the US having the largest heat map.
Sending a message during this time will results in a failed “Message not delivered” notice. It’s only a matter of time before Hangouts messaging is restored and all those pending messages finally go through.
Hopefully it wont be too much longer now but those who were using Hangouts to send SMS messages over Google Voice are arguably even more inconvenienced by the outage.
Hang in there.