I am not able to boot my computer from hard disk. whenever it wants to boot computer restarts. but I can boot it using cd rom. yesterday i wrote following command which i think made such problem
fdis /dev/hdb7
then I pressed help and then I pressed n
then I had 4 options:1,2,3,4
but I didn't choose any of them I pressed only q to exit but it didn't work and after that I pressed ctrl+z to exit
has anyone ever had such problem?

I even cannot see grub at the begining

I'm not sure if this works in FC3, but I tried it on my FC4:

1. Boot your FC3 using the CD, and type 'linux rescue' at the boot prompt (boot: linux rescue) or you can read the instruction by pressing F5
2. The installer will try to configure the rescue system, and It will mount your FC3
3. After you're logged in as root, reinstall grub, use 'grub-install /dev/hda', assuming /dev/hda is your primary hardisk.

This will reinstall grub bootloader, then reboot your machine. Hope this helps

I write this command but it says that /sbin/grub : Not found

Well Hi there,
to solve ur problem do the following:
1) Insert the Rescue CD into the CD Rom (Boot from it)
2) At the command line enter "linux rescue"
3) Use the Change Root command to fool the CD to install the Grub on your Hard Disk not on your CD (Dummy Idea), "chroot /mnt/sysimage"
4) Now all you have to do is re-install the Grub Boot Loader again do that like this:
grub-install /dev/hda (This assumes that you are using 1 HDD and the Linux is on it)
5) Reboot if the system also keeps restarting then u might be calling the Run level 6.

If the problems stil exit just make a post and i shall help you kill it
he he he he he