in the console mode or init 3 you can try this command  first

#xorgconfig

answer each questions appear on the screen. for the horizontal and vertical you may have to take a look to your monitor label. usually at the back of your monitor.  this is very important if the xorg doesnt configure it automatically. Especially in slackware. ubuntu can do this automatically

or you may try this command

#xorgsetup

it’s in gui mode. u have to fill all the questions appear.

but sometime those two command not work, so you have to take a look the xorg.conf manually.
still in init 3 mode, edit the xorg.conf

#nano /etc/X11/xorg.conf

see if the driver in device screen is the right driver. vesa is a universal driver but sometime it didnt work 100%. so if you have nvidia or ati graphic card, change the driver to nv or ati.  see the next line for configuration i have made

Driver “nv”
BusID “PCI:1:0:0”
VideoRam “128”

in the screen section make sure the device is the same with monitor section.
like “My Video Card” in screen section must be the same with monitor section device “My Video Card”

dont forget the default depth

DefaultDepth 24

save your xorg.conf