Why won't my Macbook remember any Messages chats anymore?

Can't figure out what's going on here, but my Macbook Pro (mid-2011, macOS Sierra) no longer remembers anything new in Messages. Every time I quit and relaunch Messages, the history is right back where it was before I launched. The last messages I have there are from Feb 4 (4 days ago) and anything since then is always gone. I can open it right now, chat with someone, then quit & relaunch and everything we had said since launching the app, is gone. I did just do an OS reinstall recently to try and fix some other random weird issues I was having, also repaired all permissions etc. though there was nothing to repair. I'm stumped.

EDIT: Additionally, now every time I launch Messages, my own chat (with my contact) is defaulting to an alternate email address of mine, which is not iMessages enabled, so appears in red. I can change it to my valid address, send messages do whatever I want... but again once I relaunch, it's reset to as it was before, with all new chats gone and seto to use a non-enabled email address.

You need to click on the word Messages at the top of the screen (next to the Apple Logo) and then click on preferences and then Accounts. You need to click on your iMessage account (iCloud) and then sign back in to your account to reactivate the account.

This was HUGELY obscure but it appears I've solved it. If it stays solved for a few days, I'll accept this as the answer. It involves one theory, and a number of actions.

My theory as to what happened to cause all this in the first place:

A few weeks back I was troubleshooting some other issues, which had me swap SIM cards between our phones. After swapping back, everything appeared to be fine, but I believe that in the background of iCloud somehow, it got scrambled as to whose phone belonged to which account.

This morning, I logged into my account on icloud.com and saw that all my various devices were properly listed as I would expect. Then I logged into my wife's iCloud.com account, and to my shock I saw my devices listed under her account! And none of her devices!

I promptly deleted all my devices from her account and made sure that "Find my (device)" was enabled on all her devices - it was.

Then in the evening when I had access to her phone again, I checked her iCloud account on the phone itself, and there were two iPhone SEs listed, with different phone names (the device name you can set in Settings). I deleted the "stray" phone from her account, and things immediately appeared to go back to normal again. My Macbook's chat history now opens properly and as expected, and also a separate (or so I thought) problem I was having, seems to now have been corrected. Messages sent from any of my devices to her, appear only once in a single chat on her side, and messages sent from her to me, also appear in only a single chat.

No idea how this happened, or even if my theory is correct, but at least the problem seems to be solved for now. Will see how it does for a few days before I accept this as the answer.