Unable to Boot Linux

This problem is often caused by the installation of another operating system after you have installed Red Hat Linux. Some other operating systems assume that you have no other operating systems on your computer, and they overwrite the Master Boot Record (MBR) that originally contained the GRUB or LILO boot loader. If the boot loader is overwritten in this manner, you will not be able to boot Red Hat Linux unless you can get into rescue mode.
Another common problem occurs when using a partitioning tool to resize a partition or create a new partition from free space after installation and it changes the order of your partitions. If the partition number of your / partition changes, the boot loader will not be able to nd it to mount the partition. To x this problem, boot in rescue mode and modify /boot/grub/grub.conf if you are using GRUB or /etc/lilo.conf if you are using LILO.