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.
Best Answer
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.