I have the same problem here. I did 4 updates in 1 go, including the Thunderbolt update.
When I boot up, it gives me the option to log into my ID or Guest ID. If I log into mine, it goes to the grey loading screen but stays there. If I choose the guest account, it reboots to the guest page where only Safari is open.
I'm currently logged into that now and it works fine.
I have a 2011 iMac 27" and purchased the lastest OSX from the app store. I don't have any backups or disks.
Many people are talking about re-installing the OS to fix the problem. I need to know if this will overight, delete, erase or alter any files, settings, accounts etc. It would be disasterous to lose anything on my PC.
I'm pretty sure I'll be buying the entended Apple Care and Time Capsule as soon as I can afford it but there's no point if all my data is lost.
I do have 2 external drives and a MacBook Air on hand if there's a way of backing it up.
If anyone can help, I would be overjoyed.
Sorry for the novel, just really worried about this.
Ben.