using an external source. OS X installation disk or thumb drive.
Run the disk utilities and Repair Permissions.
Alternatively, you "may" be able to run Disk Utilities from the internal drive, assuming you can actually get into the thing.
What messages do you get when you try to log in?
I recently set my ( shared & permissions ) to no one on my MacBook Pro like an idiot. Now I can't get into my laptop. Please help this dummie.