screen will not turn on at startup

[Hazem Sandouka]

hello,

I just installed YDL 3.0 on an Imac(1998, the one that requires a txt 
installer) i booted into gnome and ran the yum -y update command wish 
completed successfully.  Then i rebooted but the screen wont turn on 
anymore.  i tried reinstalling YDL even then the screen wont come on so 
i cant access the YDL installer.  I put an OS X installer cd in and when 
the installer booted the screen came on.  Anybody know what is wrong?

Thanks for ur help