Question: I cannot send Messages from my mac
I've always been able to send iMessages from my mac until just yesterday. New messages can be seen on the mac but I can no longer respond to them. When I type in the message I get an error message "Not Delivered" along with a red exclamation mark in a circle. I've quit and re-started the app and looked online but can't find any helpful hints that work. I am running High Sierra Version 10.13.3. and is completely up to date. Any suggestions?
iMac, iOS 11.2.6