If you upgrade to 10.8 or Mountain Lion, make sure you watch for the “automatic password at login” check box. Mountain Lion requires that you have a password on your Mac, however if you did not set one prior to the Mountain Lion upgrade and you miss the step of making sure the “automatic password at login box” is unchecked you will have issues logging into your computer.
If you forgot to make sure that this option was unchecked there is an easy resolution to this issue.
You can go to restart hold down command + R and it will launch the recovery console. Then you can go to the Tools drop down menu and choose terminal, at the bash hashtag, type resetpassword, click enter and highlight the Mac HD.
Choose the correct user for the account that you want and then reset password, save it and restart your computer, after that you will be able to login to your Mac.
Give us a call if you need help with any of these steps!