there's not much info to go on from your post, but you can try to make sure everything is up to date by doing
Code:
su -c "apt-get update ; apt-get dist-upgrade"
try using debian's xfree86 configuration tool.
Code:
su -c "dpkg-reconfigure -Plow xfree86"
take a look at your "/etc/X11/XF86Config-4" file and make sure that your monitor can handle the resolutions listed near the end.
if none of that works, post your XF86Config-4 file here to examine.
good luck.