I am having a similar problem. Yesterday, when I upgraded to Mavericks everything worked fine. Today Messages does not work. When I put in the person's name it appears in red and I am told that I am not signed in to my iMessage account but when I go in to the preferences it shows that I am signed in. I have tried signing out and signing back in, restarting the computer and resetting the NVRAM. Messages is working just fine from my iOS devices and from my iMac but not from my Mac Book Pro. The iMac and the Mac Book Pro are on the same network.
Any thoughts or suggestions?
Hi. I had the same weird issue. Messages suddenly stopped working - asking me for the password, which it would not accept, and the spinning pinwheel was there forever.
I tried a few things that have been suggested on various forums.
After updating from Mavericks to Yosemite, the problem persisted.
Finally, I tried resetting the PRAM but that didn't solve it.
I was using Maps a few days ago, and wished to share a location to my phone. I went to the "share" option in the drop down menu, and saw that there was an option to use Messages to send it.
I chose this option in case it would be another way in to Messages sign in etc. Long shot. Then came a prompt that said something like "no Messages account is configured" or similar. I followed the screen prompts and was presented with a login box. I entered my details and then couldn't believe it - Messages was up and running again on the Mac.
I've no idea whether this will work on anyone else's system but thought I'd share it just in case it was worth a shot.
No Messages on the Mac was really doing my nut in, as I use it regularly to transfer useful stuff quickly to the phone.