iMessage failure on iMac

Hello,

I have used iMessage seamlessly for months.

As of 2 days ago however, it worked intermittently, now not at all. "not delivered" error message - red exclamation point in circle.

I checked settings in the iPhone (turned on SMS although never needed to previously), checked passwords, etc.

I have never "needed" to enter passwords upon login but now it requires.

Also, I updated the iPhone version just the other day, wonder if this is the issue?
Thanks in advance!