I have seen several instances where someone could not login using the GUI. The system appears to be accepting the user name and password combination but returns to the GUI login screen rather than proceeding to the desktop.
I have seen this caused by a full /tmp filesystem. It could also be that you don’t have a /tmp filesystem so the root filesystem / could be full.
The problem occurs because there is no room for the X Window system to create files in needs in /tmp. You should be able to login using one of the virtual terminals because a text mode login does not create files in /tmp. Simply press the key combination Ctrl-Alt-F2( or 3,4,5 or 6) to get to a different virtual terminal login session and you can login from there.
Use the df command which will tell you the status of the filesystems. It will be necessary to delete some files from the full filesystem in order to make room. On a system using Logical Volume Manager, it may be possible to add more space to the /tmp file system to provide additional room.