It looks like a LILO configuration problem.
Did you create a boot disk? did you try with it? if it work, once you log into mandrake, you will have to recreate a Lilo configuration. If it doesn't work, it seems that the install progam does not locate the kernel.
When you finish to install the mandrake 9.1, you have a sum up of the installation and can change some points, Check for booting or Start up part, here you will be allowed to check the LILO configuration.