Login Error - Unable to perform switch user from GUI RHEL 6

Issue

Switch user option in the GUI interface of locked screen does not work and following error message appears:

Solution

1: Reboot the system in runlevel 5
2: Manually start the gdm using below command.

# gdm


Root Cause

1: System is running in runlevel 3.
2: Gui started using startx command, which does not initialize the display Manager