Just about every time that we have customers call saying that their documents, settings and Desktop on a Mac have disappeared and gone back to the Apple default, there is one cause: The user has renamed their User folder in the Finder.
One rule with computers is: Do not move, rename or delete any of the folders that the operating system creates. If you do, unexpected things will happen, or the operating system will stop working.
In this case what happens is if you rename your User Home folder from, say, Frederick to Fred, the next time you restart and log in as Frederick, the OS will say “Hey, there’s no Home folder named Frederick, I better create one.” and you end up with a new User folder with nothing in it. It looks like all your files have gone — but they haven’t. All your documents, emails, photos, music, Desktop pictures, Dock settings etc. are still in the folder which is now named Fred.
To get your documents back, make note of exactly how the new blank Frederick folder is named, rename the new Frederick folder to Oops and then rename the Fred folder to the exact name of the Frederick folder that was created. Reboot the machine. In 99% of cases this will put you back to where you want to be.
In a few cases, other folders have been moved or renamed, and you have to do more extensive repairs http://www.thexlab.com/faqs/renamehomerecovery.html
Sometimes, User folders and Library folders can be hidden:
http://support.apple.com/kb/TS1526
http://reviews.cnet.com/8301-13727_7-20082044-263/os-x-lion-where-did-my-library-go/
http://osxdaily.com/2011/08/17/mac-os-x-10-7-1-hides-user-library-folder-again/
If you really, really want to rename the username of an existing user on a Mac, follow these instructions to the letter. Do not proceed if you don’t understand the instruction. http://support.apple.com/kb/HT1428?viewlocale=en_US
If you don’t know where the User folder is (if it has gotten moved) then do a Find (Command – F) or a Spotlight search for a filename that you know is in the Documents.
In the case where a user account has been deleted and doesn’t show up in the System Preferences: Users, you can re-link an account with the User folder
For 10.6 and earlier http://www.burramundi.com/2049