This is another problem which at least I faced pretty regularly. I have a monitor that can support a resolution of 1024x768 pixels. But the setup detected the resolution wrong as a result I had a resolution of 800x600. Apart from that sometimes a few of my friends tried increasing the resolution of the monitor to a point which the monitor could not support as as result X windows would not start up.
All the remedies to these maladies lie in the X windows configuration file. It can be found in the "/etc/X11" directory with the name "XF86Config*"(use reg ex) . Open the file using your favourite text editor.
Search for a section called "Screen". Once found add a subsection after the "DefaultDepth" entry. As follows (the entry given here is for me make modification to suit your hardware).
SubSection "Display"
Depth 24
Modes "1024x768"
EndSubSection
restart X windows.
If you can't get to text mode because the xserver does not let you get there in that case you can get into run level 3(text multiuser mode) from the the grub prompt. To do this as soon as you get the GRUB screen select the line which you want to boot and press 'e'
This will take you to a screen where the first line will be the partition which will be booted. The next line will generally be the kernel which be booted. Select this line again and press 'e' you to a prompt with the same line. Here get to the end of the line and type '3' and hit enter. You will be taken to the previous screen. Now press "b" to boot. Then login as root and modify the XF86Config file.