Open up MCC ( Mandriva Control Centre ) click the Hardware tab, and then click 'Setup the graphical server'.
Make sure you have the correct graphics card and monitor selected. Press the Test button to see if your new selection will work. You get a Yes No option. If you don't see anything during the 'test', don't do anything. It is a timed test, and will go back to the last known working settings.
If that does not work, you will have to post the type of graphics card you have. Then we'll look into the driver you have loaded.