I would be concerned too if I was not able to log into my account my MacBook Pro. I recommend following the steps in this article to troubleshoot an issue like this:
If you get to the section titled "Start from your Mac OS X Install disc; use Disk Utility," you will need to shut down and restart holding command and R to reach the Recovery Partition on your MacBook Pro. Mountain Lion does not come with an installation disk, but you can use the Recovery Partition in place of an installation disk. You can learn more about the Recovery Partition in the following links:
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Jonny b Goode wrote:
When it restarts, the user logon screen comes up and i enter my user name and password and then hit enter. It accepts it and then a grey screen comes up and just hangs there.
If this was a problem with OS X or a thrid party at boot kext file, it likely would show up BEFORE the login screen.
Since it appears after the log in screen, that would suggest it's a problem with a auto-launch of a program after the account is logged into.
This is a new resume feature of OS X Lions and it's a problem obviously because you don't know what program is responsible.
At least with manual launching of programs, if the machine crashes you know what program caused it.
What you do is hold the Shift Key down upon boot, this enters Safe Mode and disables these at launch programs which you then go around and update all your third party software.
Disable the "resume windows" feature in System Preferences or use the free TinkerTool until your problem is resolved.
Apple menu > System Preferences > General >
Uncheck “Restore windows when quitting and re-opening apps“
You can look at the Console > System Diagnostic log file for a clue what program is crashing the machine.
Once you discover what it is, use the developers uninstall method to remove the program if there is no update available.
To be systematic on repairing your machine look at this User Tip,
Once we have some idea of what program is causing your machine to crash then it can be replaced with a new copy or sometimes it's the preference file for it has gotten corrupted or a file it's loading and that causes everything to crash once you log in.
If replacing the program with a new copy doesn't fix it, then it's likely it's preference file has gotten corupted and that's not reinstalled fresh when you reinstall the program, it's carried over from the previous install/use of the program.
See Step #12/#13 for details.