Messages Not Showing Up After iMac Update Restart

ETA: After everything below, after closing and re-opening Messages, messages reappeared. Problem solved. Sorry for any bother!

This morning after restarting my iMac (prompted by an update), my Messages are all gone. Left column in Messages is totally empty.

So I went into Messages > Preferences > Settings, and as a guess, checked box "Enable Messages in iCloud."

For a few minutes, the blue progress bar at bottom left was working, first "Downloading from iCloud" and then "Uploading to iCloud." This all lasted about 10-15 minutes.

Now: No more progress bar activity, but also no messages.

Just now: Tried clicking "Sync Now" beside "Enable Messages." No change.

Just just now: Got a text notification (the little badgey thing that bounces in from the right on my iMac) from someone, but still nothing in Messages. (There's even a little red notification numeral "1" in the Message icon in my Dock, but still nothing in Messages itself.)

Messages are still on my other devices (iPhone, iPad) showing up normally.

iMac w/Mac OS 10.15.7 Catalina

Any help appreciated. I'm sure it's a crashingly obvious fix. :)

Tom