Lost Message Conversations
I was out of the country and away from my desktop computer for a little over a month. When I returned and turned the desktop back on, I was asked to sign in to messages with my Apple ID and password. When I did, all my conversations were gone. I still have them on my laptop (which I used daily), but they are absent on my desktop.
New messages come in just fine, and one or two conversations that were on my desktop before came back when the person sent a new text in that conversation. The rest are not there.
What can I do to restore these conversations?
iMac, OS X El Capitan (10.11.6), Retina 5K