If you are experience an error like: "The trust relationship between this workstation and the primary domain failed." or "There are currently no log on servers available to service the logon request." Then please see this article: Trust Relationship Issues & No Log on Servers
If it seems like all you can do is log into another user, look for a "switch user" button on the bottom of the screen.
Be sure you are typing in your password correctly.
If you are still experiencing a log on problem, try restarting your computer. If you can't power off or restart from the log on screen, try pressing the power button till the computer completely powers off. After about 10 seconds, turn the computer back on. This may resolve your log on issue.
If none of these resolve the issue for you please send the helpdesk a ticket.