Boot from your installation CD, #1 I think.
When you get to a screen type 'linux rescue' <enter>
You can choose to auto mount your partitions or manually mount them,
without more detail or an exact problem, all I can say is edit /etc/inittab on your root partition and change the default runlevel to 3 .
That will allow you to boot the system to the console, and fix whatever is wrong.