the first thing is to see if you need any updates
do a text only boot ( int 3 )
log in as root and run
then make sure you have the FULL install of gnome
from the list outputed by this command
--- a very long list of what software groups are installed and NOT installed ---
--- should be something like this that needs to be installed ( or parts of them are missing )
yum groupinstall "Desktop" "Genral Purpose Desktop"