Check in the "User & Groups" System Preferences, it looks like OS X may have somehow forgotten the path to where your "Home directory:" is at.
If you can still login, verify that your Path is still what you set in System Preferences for your "Home directory:" path.
If you can't log in, then try logging in as another
Admin user to fix your home directory location.
Try to login as that user and then edit, verify where the OS thinks your home directory is for your primary user account.
Create a new Mac OS X adminstrator account without an existing administrator password might be helpful in your situation, so that you can have another admin account to help you trouble shoot.
Restart the computer in Mac OS X single user mode by holding Command-S at startup.
Mount the computer's drive for read/write access. To do so, type the following command at the command prompt, and then press Enter:
% mount -uw /
Remove the file that identifies that the initial run of the "Mac OS X Setup Assistant" has been completed, with this command at the command prompt, followed by Enter:
% rm /var/db/.applesetupdone #Update on Lion it looks like its .AppleSetupDone
Restart the computer by entering this command, followed by Enter.
The Mac OS X Setup Assistant screen should appear after the reboot, just as it does when you start up a brand new computer or upgrade to a new version of Mac OS X.
Create a new user account with the Setup Assistant. Be sure to name this user something different than the admin user that already exists on the system, and use a strong password (see "Mac OS X password tips"). At the end of the Setup Assistant process, the system will automatically log into this new user account. The account will have a unique ID (UID) that is one higher than the last user that was on the system, and will have all administration privileges.
Log in as the new user, and then edit the "Home directory:" Advanced Options path for your primary user.