I am new to Linux and most recently I installed RHEL 3 on my hp pavillion ZV5000, I couldn't get a complete display on my monitor and when I checked x windows my monitor was detected as unprobed, so I selected the "Generic LCD 1024 X 728". Which, i was promted to log out for settings to be in effect. After logging out, I can hardly see any display on my monitor (it's all dark). I really don't wish to re-install my system yet, becos I have got some important files that I recently transferred there. Pls can someone at least give me a clue that I can apply to restore it to it's former settings?

You certainly won't need to re-install anything to overcome this problem. Have a look in your /etc/X11 folder for anything resembling a backup of your X config file (the 'live' file is called either xorg.conf or XF86Config-4, depending what flavour of X RHEL uses - so a backup might be called xorg.conf.bak, ~xorg.conf etc.).

If you find a backup, you can just back up the live version and replace it with the backup. Otherwise, you can try running xconfig (possibly xf86config) to regenerate your config file in response to some questions.

someone told me a long time ago to reinstall grub not linux, Im not sure how

I can't quite see the relevance... grub is a bootloader whose sole job is to get Linux running from a BIOS environment. If you can get the kernel working, you have grub working properly and you don't need to change it.

It certainly has nothing to do with X windows configuration.