iMac Lion not logging in
The first step in troubleshooting this problem is to boot into Safe Mode.

Shut your Mac down, and press the power button. As soon as you hear the startup sound, hold down the left Shift key. Keep holding it down until the spinning clock appears.

Starting up will take longer than normal because the Mac is performing some basic disk repair procedures.

Once startup is complete, and you are back to the Finder, reboot the Mac again.

If this does not solve the problem, it is time to try the next step.