pls describe your problem a little more.
does your monitor shows this error?? if yes, you have selected an unsupported resolution or refresh rate. normally, a 15 inch monitor can't work at higher resolutions than 1024 X 768 @ 60Hz. Run Xconf (or what ever Xsetup) and select a lower resolution or refresh rate.
Have look here and try it.
What is the max. frequency your monitor supports?
You will then have to change the display settings using YAST.
Have a look here too.
Sorry I can't be very specific cos its been quite a while since I used Suse so can't recollect it very well.
You will have to reboot in safe mode and set your monitor frequency to supported settings and many times updating the drivers too helps.