I think the latest software update is messing up the Thunderbolt Display
I have owned the Thunderbolt display since it came out in 2011. I have a late 2013 MBP that has been perfect until the latest software updates. 10.10.5 just doesn't work with the monitor.... i don't know why. I can't log in as you have said. Monitor goes black and I have to reboot... I also lost the FireWire 800 port... This has to be a software issue... because that's the ony thing that changed
MacBook Pro with Retina display, OS X Yosemite (10.10.5)