Suddenly Cannot Sign In to Messages on MacBook Pro
I can't figure this out. I was, literally, using Messages on my Mac earlier this morning, without issue. Now, however, it keeps asking me for my iCloud password and when I enter it, it just sits there as though I haven't entered anything! I tried going into the Messages Preferences and entering the password there (for some reason, it has disappeared!), but then I get this error message:
I HAVE checked my network connection. My password is correct, my internet signal strength is very good, my Macbook's date and time are correct, and there have been NO system updates in the last 24 hours. What is going on??? I checked Apple's System Status Page, but iMessage is not experiencing an outage. I can't figure out what has changed in, literally, a matter of minutes. It was working just fine and all of a sudden, it's not. I was sending messages ON my Macbook Pro a couple of hours ago. I haven't downloaded anything or gone to any sketchy sites or clicked on any links. This is truly weird.
Any ideas??
2015 MacBook Pro laptop (Retina, 15"), running Mojave version 10.14.5
MacBook