Cannot log into MAC OS X after rename the disk
You can't log into your account because the OS can't find your user directory.
You have to boot the computer keeping pressed command + R to enter the restoration/repair mode. Then go to the "Utilities" in the upper menu and enter "Terminal".

Type:
/usr/sbin/diskutil rename "oldname" "newname" where oldname is the name you assigned to the disk and newname should be "Macintosh HD".

If everything is OK you will receive a message informing that the disk was renamed and you can restart your computer and log into your account.