at the command line type:
startx &>/tmp/Xerrors <--this syntax assumes you're using bash. For csh, transpose the & and >
This will start X, but will also create a file in /tmp called Xerrors, and write the standard output and standard error of the startx command to that file. To view the messages and errors that get logged to the file, type:
You can use the up/down arrows and pg up/ pg down keys to navigate; see if there more definitive clues there.
In Mandrake, you can type one of the following commands to configure X (you'll need the specs for your video card and monitor):
If neither of those work, post the specific errors you get, as well as your system/hardware specs.