If you click the iMessage icon, go the messages menu at the top of the screen, preferences/accounts you should be able to log back in there. Hope this helps anything else just ask!
I am having problems with all of my contacts showing up at "offline", on both of my macs.. Only after the most recent update, maybe a month or 2. I am signed in and says I am online, has anyone heard of this recently? Or know how to fix it? Hope all is well.