I've seen this happen a few times under different circumstances but it is down to not getting connection to a domain controller.
The two circumstances I've seen it were:
1) Wireless machines that were not getting a IP address in time during startup and did not have an active network connection by the time they got to the logon screen. Static IP address for the laptops fixed that one.
2) At a previous employer who only had 2 domain controllers. If two or three labs got turned on at 9 am it flooded the DCs with logon requests some of which went unanswered.
I'd log onto the machine localy and check you have network connection and can reach the domain controllers. I'd also take a look at the logs on the domain controllers an see if they are getting / processing logon requests.