Hi,
Bit difficult to answer without info on your partitioning. I assume you are booting from lilo. They way it works is that the binary loader has the root device inside (e.g /dev/sda1) and will attempt to mount that as mount point '/'. If the kernel needs special drivers, e.g. for scsi, it will firstly load these using the 'initrd' image. The main start-up script is /etc/rc.d/sysinit and it then looks in /etc/inittab for the default runlevel. It then mounts other partitions in /etc/fstab, and continues start-up dependent on runlevel..
The first thing to try is to provide the run-level at boot time - when you get the 'boot:' prompt at the bottom of the screen type 'linux 3' so that it looks like 'boot: linux 3' then press enter. Hopefully, that should boot the system up to runlevel 3 (multi-user console mode) and you could fix your inittab if that's become corrupted.
Next to try would be 'linux 1' (as above). This is single (root) user maintenance mode. From here you'll have to mount things by hand if they're on different partitions. Look in /etc/fstab for the details.
You may need to run 'fsck' on your partitions if there has been some damage - e.g. 'fsck /dev/sda1'.
Rgds