Audio Message: Your computer has been shut down . .

Sounds like malware to me in combination with bogus "scare tactic" virus alerts from a website.

You can install MalwareBytes to remove malware. The scary website alerts are a scam; ignore them.

https://www.malwarebytes.com/mac

Your browsers Home and default opening website may have been changed by malware. They can be changed in the app's settings.

KxR84z wrote:

Using MacBook Pro latest iOS. Was in Safari, typed in “Facebook” in browser. Clicked. Did not double check address before clicking. Entered User ID. Entered p/w. Received numeric code re: 2 step verification. Entered it.

I hope you did not type your user name and password into a scam site. If you think you may have done that, then change your password for Facebook immediately. If you used that same password for any other account, change the passwords for the other compromised accounts, too.

Home screen changed to a blue screen with a few desktop app icons (not mine). Another screen on top was flashing a message that looked like it was related to a PC.

An audio message repeatedly stated my computer was _____ (don’t recall, perhaps shut down), because someone had used my IP address to access a site with dense virus ______.

Scam!

Prior to attempting to logon to Facebook, I was on Safari and entered “Baltimore Jewish Rummage,” which I’d never visited, and forgot was told was a Facebook group.

I have no idea whether that's a legitimate Facebook group or not. But whether you are using search engines like Google, or typing URLs, it's very easy to make a mistake and go to the wrong site. Whether that's a legitimate site, a site run by a domain squatter hoping to be paid off, or a site run by a scammer.

Repetitive audio message said not to shut down computer or restart. It flashed a phone number and the word Microsoft was in the flashing screen message. I closed the laptop.

What now?

Don't call the number. That phone number almost certainly will connect you to criminals who will try to scam you out of the contents of your bank accounts (one way or the other).