cannot login to Macboo Pro

I'm f you're using Mac OS X 10.2 or later, you can start up your computer in Safe Mode, which includes an automatic disk check and repair. Hold down the shift key on start up. If this works follow these procedures:

http://support.apple.com/kb/TS1417

Hope this helps

Prohibitory symbol

When you see a circle with a slash symbol instead of the Apple logo, it means your Mac couldn't find a valid System Folder to start up from.

If you're using your Mac at a school or business, it might be trying to start from the wrong version of OS X. Contact your IT department for more help.

If this is your personal Mac, try reinstalling OS X by using OS X Recovery.