Sounds like either an incomplete installation, or else linux is having trouble reading your hard disk. An easy way to check would be to reinstall mandrake, but if you don't want to do that, try this.
If you have access to windows, use a tool like explore2fs (You will need to use rfstool instead if you opted for reiserfs partitions). Browse through the linux partitions it finds, and see if one of them has the file /etc/init. If no such file exists, you have a corrupted installation and you need to reinstall mandrake.
If the file
does exist, try booting with
linux init=/etc/init
If this fails, the only other guess I can make is that linux is trying to mount the wrong partition as its root partition. Note down the name of the partition which contains the file (like hdb1, hda5, hdb2 etc), and when your boot options menu comes up, type
Replace hdb1 with the name of the partition.
(If no command line interface is visible in your boot menu, press
c to get it).
One more thing, is any other error/fatal warning displayed when starting linux?