iMessages no longer working after Yosemite upgrade
After an hour long call with Apple, I was finally told to reset my default Keychain, and after a reboot, that worked. We diagnosed this as the issue because every time I rebooted, iCloud/FaceTime/Messages would ask for my password again, even though it had supposedly saved it.

This also came right after I had just reset the default Keychain to solve not being able to connect to my wireless network.